test: cover lyrics, transcode, and recommendation

DB-free unit tests for the three previously-untested features:
- lyrics: get-or-fetch caching, not_found TTL, force refetch, graceful miss;
  plus the LRC -> structured-lyrics serializer (timing, plain fallback, empty)
- transcode: path helpers, segment-name traversal guard, cache hit/miss, the
  unknown/not-downloaded 404 paths
- reco: ML path (order-preserving batched hydration) + metadata fallback for
  similar/radio, exclude handling, reason codes

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

alembic/versions/20260728_1100-sync_ingestion_columns.py

@@ -0,0 +1,57 @@
+"""sync: server-ingestion columns on the event logs
+
+Revision ID: 20260728_sync_synced_at
+Revises: 20260728_user_settings
+Create Date: 2026-07-28 11:00:00.000000
+
+Adds ``synced_at`` to ``likes`` and ``play_history`` — the server-side ingestion
+time used as the delta-sync ordering key (distinct from the event time
+``created_at``/``played_at``, which a sync push preserves from the client even
+when the event happened offline earlier). Existing rows are backfilled from
+their event time so a first sync after upgrade behaves sensibly.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "20260728_sync_synced_at"
+down_revision: str | None = "20260728_user_settings"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "likes",
+        sa.Column(
+            "synced_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            nullable=False,
+        ),
+    )
+    op.execute("UPDATE likes SET synced_at = created_at")
+    op.create_index(op.f("ix_likes_synced_at"), "likes", ["synced_at"])
+
+    op.add_column(
+        "play_history",
+        sa.Column(
+            "synced_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            nullable=False,
+        ),
+    )
+    op.execute("UPDATE play_history SET synced_at = played_at")
+    op.create_index(op.f("ix_play_history_synced_at"), "play_history", ["synced_at"])
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_play_history_synced_at"), table_name="play_history")
+    op.drop_column("play_history", "synced_at")
+    op.drop_index(op.f("ix_likes_synced_at"), table_name="likes")
+    op.drop_column("likes", "synced_at")

app/api/deps.py

@@ -6,20 +6,26 @@ bound to the request-scoped DB session; stateless adapters (hasher, token
 service) are process-cached.
 """
 
+import datetime as dt
 from collections.abc import AsyncIterator
 from functools import lru_cache
 from typing import Annotated
 
 from fastapi import Depends, Query
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
+from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.application.auth_service import AuthService
 from app.application.download_service import DownloadService
+from app.application.lyrics_service import LyricsService
 from app.application.metadata_service import MetadataEnrichmentService
+from app.application.recommendation_service import RecommendationService
 from app.application.remote_library_service import RemoteLibraryService
 from app.application.streaming_service import StreamingService
 from app.application.subsonic_auth_service import SubsonicAuthService
+from app.application.sync_service import SyncService
+from app.application.transcode_service import TranscodeService
 from app.application.upload_service import UploadService
 from app.application.user_service import UserService
 from app.application.user_settings_service import UserSettingsService
@@ -35,6 +41,7 @@ from app.infrastructure.db.repositories import (
     SqlAlchemyDownloadJobRepository,
     SqlAlchemyHistoryRepository,
     SqlAlchemyLikeRepository,
+    SqlAlchemyLyricsRepository,
     SqlAlchemyPlaylistRepository,
     SqlAlchemyRefreshTokenRepository,
     SqlAlchemyTrackRepository,
@@ -43,7 +50,9 @@ from app.infrastructure.db.repositories import (
 )
 from app.infrastructure.metadata.acoustid import AcoustIdHttpClient
 from app.infrastructure.metadata.fingerprint import FpcalcFingerprinter
+from app.infrastructure.metadata.lrclib import LrclibHttpClient
 from app.infrastructure.metadata.tags import MutagenTagReader
+from app.infrastructure.ml.recommender import NullRecommender
 from app.infrastructure.sources.registry import SourceRegistry, build_source_registry
 from app.infrastructure.storage.provider import get_file_storage
 from app.workers.queue import enqueue_download, enqueue_enrich, enqueue_materialize
@@ -172,6 +181,40 @@ def get_metadata_service(session: SessionDep, storage: FileStorageDep) -> Metada
     )
 
 
+def get_transcode_service(session: SessionDep) -> TranscodeService:
+    """Request-side cache lookups for transcoded renditions (§6.6). Generation
+    itself runs in the ``transcode_track`` worker, never here."""
+    return TranscodeService(
+        tracks=SqlAlchemyTrackRepository(session),
+        cache_root=get_settings().transcode_cache_path,
+    )
+
+
+def get_recommendation_service(session: SessionDep) -> RecommendationService:
+    """Radio + similarity (§6.5). ML is optional and no service/contract exists
+    yet, so we wire ``NullRecommender`` — the service then uses its metadata
+    fallback. Swap in ``RemoteRecommender(ml_service_url)`` once ML lands."""
+    return RecommendationService(
+        recommender=NullRecommender(),
+        tracks=SqlAlchemyTrackRepository(session),
+        artists=SqlAlchemyArtistRepository(session),
+        likes=SqlAlchemyLikeRepository(session),
+    )
+
+
+def get_lyrics_service(session: SessionDep) -> LyricsService:
+    """Wires the LRCLIB lyrics provider + cache repo (plan §6.7). LRCLIB is
+    keyless, so this is always available; failures degrade to ``not_found``."""
+    settings = get_settings()
+    return LyricsService(
+        lyrics=SqlAlchemyLyricsRepository(session),
+        tracks=SqlAlchemyTrackRepository(session),
+        artists=SqlAlchemyArtistRepository(session),
+        albums=SqlAlchemyAlbumRepository(session),
+        provider=LrclibHttpClient(user_agent=settings.musicbrainz_user_agent),
+    )
+
+
 def get_download_service(session: SessionDep, storage: FileStorageDep) -> DownloadService:
     return DownloadService(
         jobs=SqlAlchemyDownloadJobRepository(session),
@@ -195,10 +238,33 @@ def get_remote_library_service(session: SessionDep) -> RemoteLibraryService:
 UploadServiceDep = Annotated[UploadService, Depends(get_upload_service)]
 StreamingServiceDep = Annotated[StreamingService, Depends(get_streaming_service)]
 MetadataServiceDep = Annotated[MetadataEnrichmentService, Depends(get_metadata_service)]
+LyricsServiceDep = Annotated[LyricsService, Depends(get_lyrics_service)]
+TranscodeServiceDep = Annotated[TranscodeService, Depends(get_transcode_service)]
+RecommendationServiceDep = Annotated[
+    RecommendationService, Depends(get_recommendation_service)
+]
 DownloadServiceDep = Annotated[DownloadService, Depends(get_download_service)]
 RemoteLibraryServiceDep = Annotated[RemoteLibraryService, Depends(get_remote_library_service)]
 
 
+async def _db_now(session: AsyncSession) -> dt.datetime:
+    """The database clock — the sync cursor watermark (avoids app/DB skew)."""
+    return (await session.execute(select(func.now()))).scalar_one()
+
+
+def get_sync_service(session: SessionDep) -> SyncService:
+    return SyncService(
+        likes=SqlAlchemyLikeRepository(session),
+        history=SqlAlchemyHistoryRepository(session),
+        playlists=SqlAlchemyPlaylistRepository(session),
+        tracks=SqlAlchemyTrackRepository(session),
+        now=lambda: _db_now(session),
+    )
+
+
+SyncServiceDep = Annotated[SyncService, Depends(get_sync_service)]
+
+
 # -- library repository deps ---------------------------------------------------
 def get_track_repository(session: SessionDep) -> SqlAlchemyTrackRepository:
     return SqlAlchemyTrackRepository(session)

app/api/rest/media.py

@@ -1,10 +1,11 @@
-"""Subsonic media endpoints: stream, download, cover art.
+"""Subsonic media endpoints: stream, download, cover art, lyrics.
 
 ``stream`` and ``download`` reuse :class:`StreamingService` (honouring HTTP
 Range) — they return raw bytes, not the Subsonic envelope. Transcoding params
 (``maxBitRate``/``format``) are accepted but ignored; the original file is served
-(no in-request ffmpeg — CLAUDE.md). ``getCoverArt`` returns a placeholder until
-the cover pipeline lands (the ``/api/v1`` cover endpoints are still stubs).
+(no in-request ffmpeg — CLAUDE.md). ``getCoverArt`` serves the album cover (a
+placeholder when there's none). ``getLyricsBySongId`` adapts the native
+``LyricsService`` into the OpenSubsonic structured-lyrics shape.
 """
 
 import base64
@@ -16,12 +17,17 @@ from fastapi.responses import Response, StreamingResponse
 from app.api.covers import resolve_album_for_track, stream_cover
 from app.api.deps import (
     AlbumRepoDep,
+    ArtistRepoDep,
     FileStorageDep,
+    LyricsServiceDep,
     StreamingServiceDep,
+    SubsonicFormat,
     SubsonicUser,
     TrackRepoDep,
 )
+from app.api.rest.envelope import subsonic_response
 from app.api.rest.ids import IdKind, decode_track, parse
+from app.api.rest.serializers import structured_lyrics
 from app.domain.entities.album import Album
 from app.domain.errors import NotFoundError, StorageError
 
@@ -99,6 +105,35 @@ async def get_cover_art(
     if album is not None and album.cover_path:
         try:
             return await stream_cover(storage, album.cover_path)
-        except NotFoundError, StorageError:
+        except (NotFoundError, StorageError):
             pass
     return Response(content=_PLACEHOLDER_PNG, media_type="image/png")
+
+
+@router.api_route("/getLyricsBySongId", methods=["GET", "POST"])
+@router.api_route("/getLyricsBySongId.view", methods=["GET", "POST"])
+async def get_lyrics_by_song_id(
+    _user: SubsonicUser,
+    fmt: SubsonicFormat,
+    lyrics_service: LyricsServiceDep,
+    track_repo: TrackRepoDep,
+    artist_repo: ArtistRepoDep,
+    id: Annotated[str, Query()],
+) -> Response:
+    # OpenSubsonic structured lyrics over the native LyricsService (§6.7). A miss
+    # is a normal empty ``lyricsList`` — the service degrades to not_found rather
+    # than raising, so clients get 200 either way.
+    track_id = decode_track(id)
+    track = await track_repo.get_by_id(track_id)
+    if track is None:
+        raise NotFoundError("Song not found.")
+    lyrics = await lyrics_service.get_lyrics(track_id)
+    artist = await artist_repo.get_by_id(track.artist_id)
+    return subsonic_response(
+        structured_lyrics(
+            lyrics,
+            display_artist=artist.name if artist is not None else "",
+            display_title=track.title,
+        ),
+        fmt=fmt,
+    )

app/api/rest/serializers.py

@@ -6,10 +6,17 @@ JSON equivalents). No business logic — they only reshape and rename.
 """
 
 import datetime as dt
+import re
 from typing import Any
 
 from app.api.rest.ids import encode_album, encode_artist, encode_track
 from app.domain.entities import Album, Artist, Track
+from app.domain.entities.lyrics import Lyrics
+
+# One LRC timecode: ``[mm:ss.xx]`` / ``[mm:ss.xxx]`` (fraction optional). A line
+# may carry several (the same words repeat at multiple times); metadata tags like
+# ``[ar:..]`` don't match, so they're ignored.
+_LRC_TAG_RE = re.compile(r"\[(\d+):(\d{1,2})(?:[.:](\d{1,3}))?\]")
 
 # Suffix → MIME, for the ``contentType``/``suffix`` song attributes. A
 # presentation detail (mirrors StreamingService's content-type negotiation).
@@ -90,3 +97,52 @@ def song_dict(
         "type": "music",
         "isVideo": False,
     }
+
+
+def _parse_lrc(synced: str) -> list[dict[str, Any]]:
+    """LRC text → OpenSubsonic ``line`` dicts (``start`` in ms, ``value`` text),
+    ordered by time. Lines with no timecode (blank lines, metadata tags) drop out;
+    a timecode carrying several stamps yields one line per stamp."""
+    lines: list[tuple[int, str]] = []
+    for raw in synced.splitlines():
+        stamps = list(_LRC_TAG_RE.finditer(raw))
+        if not stamps:
+            continue
+        text = _LRC_TAG_RE.sub("", raw).strip()
+        for m in stamps:
+            minutes, seconds = int(m.group(1)), int(m.group(2))
+            # LRC fractions are centiseconds (2 digits) or ms (3); pad to ms.
+            ms = int((m.group(3) or "0").ljust(3, "0")[:3])
+            lines.append(((minutes * 60 + seconds) * 1000 + ms, text))
+    lines.sort(key=lambda pair: pair[0])
+    return [{"start": start, "value": text} for start, text in lines]
+
+
+def structured_lyrics(
+    lyrics: Lyrics, *, display_artist: str, display_title: str
+) -> dict[str, Any]:
+    """OpenSubsonic ``getLyricsBySongId`` payload. Prefers synced (LRC) lines and
+    falls back to plain text; an empty ``lyricsList`` when the track has none."""
+    lines: list[dict[str, Any]] = []
+    synced = False
+    if lyrics.synced:
+        lines = _parse_lrc(lyrics.synced)
+        synced = bool(lines)
+    if not lines and lyrics.plain:
+        lines = [{"value": line} for line in lyrics.plain.splitlines()]
+    if not lines:
+        return {"lyricsList": {}}
+    return {
+        "lyricsList": {
+            "structuredLyrics": [
+                {
+                    "displayArtist": display_artist,
+                    "displayTitle": display_title,
+                    "lang": "xxx",
+                    "offset": 0,
+                    "synced": synced,
+                    "line": lines,
+                }
+            ]
+        }
+    }

app/api/schemas/lyrics.py

@@ -0,0 +1,33 @@
+"""Lyrics response schema (§6.7 / Now Playing lyrics panel).
+
+Returns the raw LRC (``synced``) and/or ``plain`` text; the client parses LRC
+timestamps for synced highlighting. A miss is a normal 200 with
+``status="not_found"`` and null text — not an error — so the panel can render a
+"no lyrics" state.
+"""
+
+import uuid
+
+from pydantic import BaseModel
+
+from app.domain.entities.lyrics import Lyrics
+
+
+class LyricsOut(BaseModel):
+    track_id: uuid.UUID
+    status: str
+    source: str | None
+    synced: str | None
+    plain: str | None
+    synced_available: bool
+
+    @classmethod
+    def from_entity(cls, lyrics: Lyrics) -> LyricsOut:
+        return cls(
+            track_id=lyrics.track_id,
+            status=lyrics.status,
+            source=lyrics.source,
+            synced=lyrics.synced,
+            plain=lyrics.plain,
+            synced_available=lyrics.synced is not None,
+        )

app/api/schemas/radio.py

@@ -0,0 +1,43 @@
+"""Radio + similarity response schemas (§6.5)."""
+
+import uuid
+
+from pydantic import BaseModel, Field
+
+from app.api.schemas.artist import ArtistOut
+from app.api.schemas.track import TrackOut
+
+
+class RadioRequest(BaseModel):
+    """Start or continue a radio. ``seed_track_id`` seeds from a track;
+    ``from_likes`` seeds from the caller's likes. ``exclude_ids`` are already-
+    queued tracks to skip (the client drives the infinite feed). ``exploration``
+    biases familiar↔new."""
+
+    seed_track_id: uuid.UUID | None = None
+    from_likes: bool = False
+    exploration: float = Field(default=0.25, ge=0.0, le=1.0)
+    count: int = Field(default=20, ge=1, le=50)
+    exclude_ids: list[uuid.UUID] = Field(default_factory=list)
+
+
+class RadioTrackOut(BaseModel):
+    track: TrackOut
+    # Short code the client localizes: ml | similar | from_likes | discover.
+    reason: str
+
+
+class RadioResponse(BaseModel):
+    # Where the picks came from: "ml" or "metadata" (fallback).
+    source: str
+    tracks: list[RadioTrackOut]
+
+
+class SimilarTracksOut(BaseModel):
+    source: str
+    tracks: list[TrackOut]
+
+
+class SimilarArtistsOut(BaseModel):
+    source: str
+    artists: list[ArtistOut]

app/api/schemas/sync.py

@@ -0,0 +1,82 @@
+"""Offline-first sync schemas (delta pull + idempotent push).
+
+The client keeps an opaque ``cursor`` (a server-clock timestamp). It pulls
+everything changed in the half-open window ``(since, cursor]`` and pushes the
+append-only events it accumulated offline. Events carry a client-generated
+``id`` so a replay is idempotent.
+"""
+
+import datetime as dt
+import uuid
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+from app.api.schemas.track import TrackOut
+
+LikeValue = Literal["like", "dislike", "neutral"]
+
+
+# -- pull (server -> client) --------------------------------------------------
+class LikeEventOut(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    value: str
+    created_at: dt.datetime
+
+
+class PlayEventOut(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    played_at: dt.datetime
+    play_duration_seconds: int | None
+    completed: bool
+
+
+class PlaylistSyncOut(BaseModel):
+    id: uuid.UUID
+    name: str
+    description: str | None
+    version: int
+    updated_at: dt.datetime
+    track_ids: list[uuid.UUID]
+
+
+class SyncChangesOut(BaseModel):
+    """Everything that changed for the caller since their last cursor. Feed
+    ``cursor`` back as ``?since=`` on the next pull."""
+
+    cursor: dt.datetime
+    likes: list[LikeEventOut]
+    plays: list[PlayEventOut]
+    playlists: list[PlaylistSyncOut]
+    tracks: list[TrackOut]
+
+
+# -- push (client -> server) --------------------------------------------------
+class LikeEventIn(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    value: LikeValue
+    created_at: dt.datetime
+
+
+class PlayEventIn(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    played_at: dt.datetime
+    play_duration_seconds: int | None = None
+    completed: bool = False
+
+
+class SyncPushIn(BaseModel):
+    likes: list[LikeEventIn] = Field(default_factory=list)
+    plays: list[PlayEventIn] = Field(default_factory=list)
+
+
+class SyncPushOut(BaseModel):
+    """How many events were newly stored (a replay reports 0) + a fresh cursor."""
+
+    cursor: dt.datetime
+    accepted_likes: int
+    accepted_plays: int

app/api/schemas/transcode.py

@@ -0,0 +1,11 @@
+"""Transcode/optimize response schemas (§6.6)."""
+
+from pydantic import BaseModel
+
+
+class OptimizeEnqueuedOut(BaseModel):
+    """Acknowledgement that a transcode job was queued (it runs in the worker)."""
+
+    status: str
+    job_id: str
+    quality: str

app/api/v1/artists.py

@@ -1,14 +1,20 @@
 """Artist endpoints."""
 
 import uuid
-from typing import Any
 
 from fastapi import APIRouter, Query
 
-from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, TrackRepoDep
+from app.api.deps import (
+    AlbumRepoDep,
+    ArtistRepoDep,
+    CurrentUser,
+    RecommendationServiceDep,
+    TrackRepoDep,
+)
 from app.api.schemas.album import AlbumOut
 from app.api.schemas.artist import ArtistOut
 from app.api.schemas.pagination import PagedResponse
+from app.api.schemas.radio import SimilarArtistsOut
 from app.api.schemas.track import TrackOut
 from app.api.v1.albums import _build_album_out
 from app.api.v1.tracks import _build_track_out
@@ -124,4 +130,24 @@ async def get_artist_tracks(
 
 
 @router.get("/{artist_id}/similar")
-async def get_similar_artists(artist_id: uuid.UUID, _: CurrentUser) -> Any: ...
+async def get_similar_artists(
+    artist_id: uuid.UUID,
+    service: RecommendationServiceDep,
+    artist_repo: ArtistRepoDep,
+    _: CurrentUser,
+    limit: int = Query(20, ge=1, le=100),
+) -> SimilarArtistsOut:
+    """Artists similar to this one (§6.5). ML when configured, else a shared-
+    genre metadata heuristic."""
+    source, artists = await service.similar_artists(artist_id, limit=limit)
+    items = [
+        ArtistOut(
+            id=a.id,
+            name=a.name,
+            album_count=await artist_repo.album_count(a.id),
+            track_count=await artist_repo.track_count(a.id),
+            created_at=a.created_at,
+        )
+        for a in artists
+    ]
+    return SimilarArtistsOut(source=source, artists=items)

app/api/v1/radio.py

@@ -1,15 +1,83 @@
-"""Radio / continuous-mix endpoints. Degrades gracefully when ML service is down."""
+"""Radio / continuous-mix endpoints (§6.5).
 
-from typing import Any
+Stateless: the client passes the seed + already-queued ids and pulls more as the
+queue drains (offline-first infinite feed). Degrades gracefully when no ML
+service is configured — the recommendation service falls back to metadata.
+"""
 
 from fastapi import APIRouter
 
+from app.api.deps import (
+    AlbumRepoDep,
+    ArtistRepoDep,
+    CurrentUser,
+    RecommendationServiceDep,
+)
+from app.api.schemas.radio import RadioRequest, RadioResponse, RadioTrackOut
+from app.api.v1.tracks import _build_track_out
+from app.application.recommendation_service import RadioPick
+
 router = APIRouter(prefix="/radio", tags=["radio"])
 
 
+async def _to_response(
+    source: str,
+    picks: list[RadioPick],
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+) -> RadioResponse:
+    tracks = [p.track for p in picks]
+    artist_ids = list({t.artist_id for t in tracks})
+    album_ids = list({t.album_id for t in tracks if t.album_id is not None})
+    artists = {a.id: a for a in await artist_repo.get_many(artist_ids)}
+    albums = {a.id: a for a in await album_repo.get_many(album_ids)}
+    outs = await _build_track_out(tracks, artists, albums)
+    return RadioResponse(
+        source=source,
+        tracks=[
+            RadioTrackOut(track=out, reason=pick.reason)
+            for out, pick in zip(outs, picks, strict=True)
+        ],
+    )
+
+
+async def _run_radio(
+    body: RadioRequest,
+    user: CurrentUser,
+    service: RecommendationServiceDep,
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+) -> RadioResponse:
+    source, picks = await service.radio(
+        user_id=user.id,
+        seed_track_id=body.seed_track_id,
+        from_likes=body.from_likes,
+        exploration=body.exploration,
+        limit=body.count,
+        exclude_ids=body.exclude_ids,
+    )
+    return await _to_response(source, picks, artist_repo, album_repo)
+
+
 @router.post("")
-async def start_radio() -> Any: ...
+async def start_radio(
+    body: RadioRequest,
+    user: CurrentUser,
+    service: RecommendationServiceDep,
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+) -> RadioResponse:
+    """Start a radio from a seed track or the caller's likes."""
+    return await _run_radio(body, user, service, artist_repo, album_repo)
 
 
 @router.post("/next")
-async def next_radio_track() -> Any: ...
+async def next_radio_track(
+    body: RadioRequest,
+    user: CurrentUser,
+    service: RecommendationServiceDep,
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+) -> RadioResponse:
+    """Fetch more tracks as the radio queue drains (pass ``exclude_ids``)."""
+    return await _run_radio(body, user, service, artist_repo, album_repo)

app/api/v1/streaming.py

@@ -1,30 +1,54 @@
-"""Audio streaming endpoint — direct stream with Range support."""
+"""Audio streaming — direct byte-range stream, transcoded quality, and HLS.
 
+``GET /stream/{id}`` streams the master with Range support, or a cached Opus
+rendition when ``?quality=`` is set (a cache miss falls back to the master and
+warms the cache in the background — playback never waits on ffmpeg). ``/hls/*``
+serves the cached HLS rendition (generated by the ``transcode_track`` worker).
+"""
+
+import re
 import uuid
 from typing import Annotated
 
-from fastapi import APIRouter, Header
-from fastapi.responses import StreamingResponse
+import anyio
+from fastapi import APIRouter, Header, Query, Response
+from fastapi.responses import FileResponse, StreamingResponse
 
-from app.api.deps import StreamingServiceDep, StreamUser
+from app.api.deps import StreamingServiceDep, StreamUser, TranscodeServiceDep
+from app.domain.errors import NotFoundError
+from app.workers.queue import enqueue_transcode_quiet
 
 router = APIRouter(prefix="/stream", tags=["streaming"])
 
+_HLS_PLAYLIST_TYPE = "application/vnd.apple.mpegurl"
+_HLS_SEGMENT_TYPE = "video/mp2t"
+_OPUS_TYPE = "audio/ogg"
+_SEGMENT_LINE_RE = re.compile(r"^(seg_\d+\.ts)$", re.MULTILINE)
+
 
 @router.get("/{track_id}")
 async def stream_track(
     track_id: uuid.UUID,
     service: StreamingServiceDep,
+    transcode: TranscodeServiceDep,
     _user: StreamUser,
     range_header: Annotated[str | None, Header(alias="Range")] = None,
-) -> StreamingResponse:
+    quality: Annotated[str | None, Query()] = None,
+) -> Response:
+    # A quality rendition, if one is cached; otherwise fall back to the master
+    # and enqueue generation so the next play gets it (graceful degradation).
+    if quality and quality != "original":
+        cached = await transcode.resolve_quality_file(track_id, quality)
+        if cached is not None:
+            return FileResponse(cached, media_type=_OPUS_TYPE)
+        await enqueue_transcode_quiet(track_id, quality=quality, hls=False)
+
     result = await service.open_stream(track_id, range_header)
 
     headers = {
         "Accept-Ranges": "bytes",
         "Content-Length": str(result.content_length),
     }
-
     if result.is_partial:
         headers["Content-Range"] = f"bytes {result.start}-{result.end}/{result.total_size}"
         status_code = 206
@@ -37,3 +61,38 @@ async def stream_track(
         headers=headers,
         media_type=result.content_type,
     )
+
+
+@router.get("/{track_id}/hls/playlist.m3u8")
+async def stream_hls_playlist(
+    track_id: uuid.UUID,
+    transcode: TranscodeServiceDep,
+    _user: StreamUser,
+    token: Annotated[str | None, Query()] = None,
+) -> Response:
+    """Serve the cached HLS playlist. On a miss, kick off generation and 404 so
+    the client retries. Segment URLs are relative; when the request carried a
+    ``?token=`` (players can't set an Authorization header), it's appended to
+    each segment line so the segment requests authenticate the same way."""
+    path = await transcode.hls_playlist(track_id)
+    if path is None:
+        await enqueue_transcode_quiet(track_id, hls=True)
+        raise NotFoundError("HLS rendition is being prepared; retry shortly.")
+
+    body = await anyio.to_thread.run_sync(path.read_text)
+    if token:
+        body = _SEGMENT_LINE_RE.sub(rf"\1?token={token}", body)
+    return Response(body, media_type=_HLS_PLAYLIST_TYPE)
+
+
+@router.get("/{track_id}/hls/{segment}")
+async def stream_hls_segment(
+    track_id: uuid.UUID,
+    segment: str,
+    transcode: TranscodeServiceDep,
+    _user: StreamUser,
+) -> FileResponse:
+    path = transcode.hls_segment(track_id, segment)
+    if path is None:
+        raise NotFoundError("Segment not found.")
+    return FileResponse(path, media_type=_HLS_SEGMENT_TYPE)

app/api/v1/sync.py

@@ -1,15 +1,103 @@
-"""Client sync endpoints (offline-first event log)."""
+"""Client sync endpoints (offline-first event log).
 
-from typing import Any
+``GET /sync/changes`` pulls everything the caller changed since their cursor;
+``POST /sync/push`` uploads the like/play events a client accumulated offline
+(idempotent — replays are no-ops). See :mod:`app.application.sync_service`.
+"""
+
+import datetime as dt
 
 from fastapi import APIRouter
 
+from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, SyncServiceDep
+from app.api.schemas.sync import (
+    LikeEventOut,
+    PlayEventOut,
+    PlaylistSyncOut,
+    SyncChangesOut,
+    SyncPushIn,
+    SyncPushOut,
+)
+from app.api.v1.tracks import _build_track_out
+from app.application.sync_service import LikeEvent, PlayEvent
+
 router = APIRouter(prefix="/sync", tags=["sync"])
 
 
 @router.get("/changes")
-async def get_changes() -> Any: ...
+async def get_changes(
+    service: SyncServiceDep,
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+    user: CurrentUser,
+    since: dt.datetime | None = None,
+) -> SyncChangesOut:
+    """Delta since ``since`` (omit for a full snapshot). Persist ``cursor`` from
+    the response and pass it back as ``?since=`` next time."""
+    changes = await service.get_changes(user.id, since=since)
+
+    artist_ids = list({t.artist_id for t in changes.tracks})
+    album_ids = list({t.album_id for t in changes.tracks if t.album_id is not None})
+    artists = {a.id: a for a in await artist_repo.get_many(artist_ids)}
+    albums = {a.id: a for a in await album_repo.get_many(album_ids)}
+    tracks_out = await _build_track_out(changes.tracks, artists, albums)
+
+    return SyncChangesOut(
+        cursor=changes.cursor,
+        likes=[
+            LikeEventOut(
+                id=lk.id, track_id=lk.track_id, value=lk.value, created_at=lk.created_at
+            )
+            for lk in changes.likes
+        ],
+        plays=[
+            PlayEventOut(
+                id=p.id,
+                track_id=p.track_id,
+                played_at=p.played_at,
+                play_duration_seconds=p.play_duration_seconds,
+                completed=p.completed,
+            )
+            for p in changes.plays
+        ],
+        playlists=[
+            PlaylistSyncOut(
+                id=d.playlist.id,
+                name=d.playlist.name,
+                description=d.playlist.description,
+                version=d.playlist.version,
+                updated_at=d.playlist.updated_at,
+                track_ids=d.track_ids,
+            )
+            for d in changes.playlists
+        ],
+        tracks=tracks_out,
+    )
 
 
 @router.post("/push")
-async def push_changes() -> Any: ...
+async def push_changes(
+    body: SyncPushIn, service: SyncServiceDep, user: CurrentUser
+) -> SyncPushOut:
+    result = await service.push(
+        user.id,
+        likes=[
+            LikeEvent(id=e.id, track_id=e.track_id, value=e.value, created_at=e.created_at)
+            for e in body.likes
+        ],
+        plays=[
+            PlayEvent(
+                id=e.id,
+                track_id=e.track_id,
+                played_at=e.played_at,
+                play_duration_seconds=e.play_duration_seconds,
+                completed=e.completed,
+            )
+            for e in body.plays
+        ],
+    )
+    return SyncPushOut(
+        cursor=result.cursor,
+        accepted_likes=result.accepted_likes,
+        accepted_plays=result.accepted_plays,
+    )

app/api/v1/tracks.py

@@ -1,8 +1,9 @@
 """Track endpoints."""
 
 import uuid
-from typing import Any
+from typing import Annotated
 
+import anyio
 from fastapi import APIRouter, Query, Response
 from fastapi.responses import StreamingResponse
 
@@ -12,13 +13,17 @@ from app.api.deps import (
     ArtistRepoDep,
     CurrentUser,
     FileStorageDep,
+    LyricsServiceDep,
     MetadataServiceDep,
+    RecommendationServiceDep,
     RemoteLibraryServiceDep,
     StreamUser,
     TrackRepoDep,
 )
 from app.api.schemas.download import DownloadJobOut
+from app.api.schemas.lyrics import LyricsOut
 from app.api.schemas.pagination import PagedResponse
+from app.api.schemas.radio import SimilarTracksOut
 from app.api.schemas.track import (
     MaterializeResponse,
     MetadataApply,
@@ -28,10 +33,13 @@ from app.api.schemas.track import (
     TrackOut,
     TrackUpdate,
 )
+from app.api.schemas.transcode import OptimizeEnqueuedOut
+from app.application.transcode_service import bitrate_for_quality, remove_track_cache
+from app.core.config import get_settings
 from app.domain.entities.album import Album
 from app.domain.entities.track import Artist, Track
-from app.domain.errors import NotFoundError
-from app.workers.queue import enqueue
+from app.domain.errors import NotFoundError, ValidationError
+from app.workers.queue import enqueue, enqueue_transcode
 
 router = APIRouter(prefix="/tracks", tags=["tracks"])
 
@@ -213,15 +221,69 @@ async def delete_track(
     await track_repo.delete(track_id)
     if track.storage_uri is not None:
         await storage.delete(track.storage_uri)
+    # Drop any cached transcode renditions (Opus + HLS) so they don't dangle.
+    await anyio.to_thread.run_sync(
+        remove_track_cache, get_settings().transcode_cache_path, track_id
+    )
     return Response(status_code=204)
 
 
 @router.get("/{track_id}/similar")
-async def get_similar_tracks(track_id: uuid.UUID, _: CurrentUser) -> Any: ...
+async def get_similar_tracks(
+    track_id: uuid.UUID,
+    service: RecommendationServiceDep,
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+    _: CurrentUser,
+    limit: Annotated[int, Query(ge=1, le=100)] = 20,
+) -> SimilarTracksOut:
+    """Tracks similar to this one (§6.5). Uses ML when configured, else a
+    genre/artist metadata heuristic."""
+    source, tracks = await service.similar_tracks(track_id, limit=limit)
+    artist_ids = list({t.artist_id for t in tracks})
+    album_ids = list({t.album_id for t in tracks if t.album_id is not None})
+    artists = {a.id: a for a in await artist_repo.get_many(artist_ids)}
+    albums = {a.id: a for a in await album_repo.get_many(album_ids)}
+    outs = await _build_track_out(tracks, artists, albums)
+    return SimilarTracksOut(source=source, tracks=outs)
 
 
-@router.post("/{track_id}/optimize")
-async def optimize_track(track_id: uuid.UUID, _: CurrentUser) -> Any: ...
+@router.post("/{track_id}/optimize", status_code=202)
+async def optimize_track(
+    track_id: uuid.UUID,
+    track_repo: TrackRepoDep,
+    _: CurrentUser,
+    quality: Annotated[str, Query()] = "high",
+) -> OptimizeEnqueuedOut:
+    """Enqueue transcoding of a track into a cached Opus rendition + HLS (§6.6).
+    Heavy ffmpeg work runs in the worker; this only queues it."""
+    if bitrate_for_quality(quality) is None:
+        raise ValidationError(
+            f"Unknown quality '{quality}'; expected one of high, medium, low."
+        )
+    track = await track_repo.get_by_id(track_id)
+    if track is None:
+        raise NotFoundError(f"Track {track_id} not found.")
+    job_id = await enqueue_transcode(track_id, quality=quality, hls=True)
+    return OptimizeEnqueuedOut(status="enqueued", job_id=job_id, quality=quality)
+
+
+@router.get("/{track_id}/lyrics")
+async def get_track_lyrics(
+    track_id: uuid.UUID, lyrics: LyricsServiceDep, _: CurrentUser
+) -> LyricsOut:
+    """Cached lyrics for the Now Playing panel (§6.7). A miss is a normal 200
+    with ``status="not_found"`` — the provider (LRCLIB) is queried at most once,
+    then the outcome is cached."""
+    return LyricsOut.from_entity(await lyrics.get_lyrics(track_id))
+
+
+@router.post("/{track_id}/lyrics/refetch")
+async def refetch_track_lyrics(
+    track_id: uuid.UUID, lyrics: LyricsServiceDep, _: CurrentUser
+) -> LyricsOut:
+    """Force a fresh provider lookup, bypassing the cache (user-triggered)."""
+    return LyricsOut.from_entity(await lyrics.get_lyrics(track_id, force=True))
 
 
 @router.get("/{track_id}/cover")

app/application/lyrics_service.py

@@ -0,0 +1,97 @@
+"""Lyrics service (plan §6.7).
+
+Get-or-fetch with caching: a track's lyrics are served from the DB when present;
+on a miss (or an expired ``not_found``) we ask the provider (LRCLIB) once, then
+cache the outcome. ``not_found`` is cached with a TTL so tracks that genuinely
+have no lyrics aren't looked up on every play, but can eventually be retried.
+
+Degrades gracefully: if the provider is unreachable the lookup just yields a
+``not_found`` — the endpoint still returns 200 with empty lyrics, never an error.
+"""
+
+import datetime as dt
+import uuid
+
+from app.domain.entities.lyrics import Lyrics
+from app.domain.errors import NotFoundError
+from app.domain.ports import (
+    AlbumRepository,
+    ArtistRepository,
+    LyricsProvider,
+    LyricsRepository,
+    TrackRepository,
+)
+
+# Re-lookup a cached "not_found" only after this long — long enough not to spam
+# the provider, short enough that lyrics added upstream eventually surface.
+_NOT_FOUND_TTL = dt.timedelta(days=7)
+
+_STATUS_FOUND = "found"
+_STATUS_NOT_FOUND = "not_found"
+
+
+class LyricsService:
+    def __init__(
+        self,
+        *,
+        lyrics: LyricsRepository,
+        tracks: TrackRepository,
+        artists: ArtistRepository,
+        albums: AlbumRepository,
+        provider: LyricsProvider,
+    ) -> None:
+        self._lyrics = lyrics
+        self._tracks = tracks
+        self._artists = artists
+        self._albums = albums
+        self._provider = provider
+
+    async def get_lyrics(self, track_id: uuid.UUID, *, force: bool = False) -> Lyrics:
+        """Return cached lyrics, fetching from the provider on a miss/expiry.
+        ``force`` (the refetch endpoint) bypasses the cache entirely."""
+        cached = await self._lyrics.get(track_id)
+        if not force and cached is not None and self._is_fresh(cached):
+            return cached
+        return await self._fetch_and_cache(track_id)
+
+    def _is_fresh(self, cached: Lyrics) -> bool:
+        if cached.status == _STATUS_FOUND:
+            return True
+        if cached.status == _STATUS_NOT_FOUND:
+            return dt.datetime.now(dt.UTC) - cached.fetched_at < _NOT_FOUND_TTL
+        # "pending" (never fetched) → not fresh, go fetch.
+        return False
+
+    async def _fetch_and_cache(self, track_id: uuid.UUID) -> Lyrics:
+        track = await self._tracks.get_by_id(track_id)
+        if track is None:
+            raise NotFoundError(f"Track {track_id} not found.")
+
+        artist = await self._artists.get_by_id(track.artist_id)
+        album = (
+            await self._albums.get_by_id(track.album_id)
+            if track.album_id is not None
+            else None
+        )
+        result = await self._provider.fetch(
+            artist=artist.name if artist else "",
+            title=track.title,
+            album=album.title if album else None,
+            duration_seconds=track.duration_seconds,
+        )
+
+        if result is None:
+            return await self._lyrics.upsert(
+                track_id=track_id,
+                synced=None,
+                plain=None,
+                source=None,
+                status=_STATUS_NOT_FOUND,
+            )
+        return await self._lyrics.upsert(
+            track_id=track_id,
+            synced=result.synced,
+            plain=result.plain,
+            source=result.source,
+            status=_STATUS_FOUND,
+        )

app/application/recommendation_service.py

@@ -0,0 +1,189 @@
+"""Recommendation / radio service (plan §6.5).
+
+Tries the external ML recommender first; when it's unavailable or declines
+(returns ``None``), falls back to metadata heuristics over the catalogue — so
+similar/radio always work, worse, without ML (graceful-degradation invariant).
+
+``reason`` values are short codes (``ml`` / ``similar`` / ``from_likes`` /
+``discover``) the client localizes for the "why is this playing?" affordance.
+"""
+
+import random
+import uuid
+from dataclasses import dataclass
+
+from app.domain.entities.track import Artist, Track
+from app.domain.errors import NotFoundError
+from app.domain.ports import (
+    ArtistRepository,
+    LikeRepository,
+    Recommender,
+    TrackRepository,
+)
+
+REASON_ML = "ml"
+REASON_SIMILAR = "similar"
+REASON_FROM_LIKES = "from_likes"
+REASON_DISCOVER = "discover"
+
+_LIKED_SEED_POOL = 50
+
+
+@dataclass(frozen=True, slots=True)
+class RadioPick:
+    track: Track
+    reason: str
+
+
+class RecommendationService:
+    def __init__(
+        self,
+        *,
+        recommender: Recommender,
+        tracks: TrackRepository,
+        artists: ArtistRepository,
+        likes: LikeRepository,
+    ) -> None:
+        self._recommender = recommender
+        self._tracks = tracks
+        self._artists = artists
+        self._likes = likes
+
+    # -- similar ---------------------------------------------------------------
+
+    async def similar_tracks(
+        self, track_id: uuid.UUID, *, limit: int
+    ) -> tuple[str, list[Track]]:
+        seed = await self._tracks.get_by_id(track_id)
+        if seed is None:
+            raise NotFoundError(f"Track {track_id} not found.")
+
+        if self._recommender.is_available():
+            ids = await self._recommender.similar_track_ids(
+                track_id, limit=limit, exclude_ids=[track_id]
+            )
+            if ids is not None:
+                return REASON_ML, await self._hydrate_tracks(ids)
+
+        found = await self._tracks.list_similar(
+            genre=seed.genre,
+            artist_id=seed.artist_id,
+            exclude_ids=[track_id],
+            limit=limit,
+        )
+        return REASON_SIMILAR, found
+
+    async def similar_artists(
+        self, artist_id: uuid.UUID, *, limit: int
+    ) -> tuple[str, list[Artist]]:
+        if await self._artists.get_by_id(artist_id) is None:
+            raise NotFoundError(f"Artist {artist_id} not found.")
+
+        if self._recommender.is_available():
+            ids = await self._recommender.similar_artist_ids(artist_id, limit=limit)
+            if ids is not None:
+                by_id = {a.id: a for a in await self._artists.get_many(ids)}
+                return REASON_ML, [by_id[i] for i in ids if i in by_id]
+
+        found = await self._artists.list_similar(artist_id=artist_id, limit=limit)
+        return REASON_SIMILAR, found
+
+    # -- radio -----------------------------------------------------------------
+
+    async def radio(
+        self,
+        *,
+        user_id: uuid.UUID,
+        seed_track_id: uuid.UUID | None,
+        from_likes: bool,
+        exploration: float,
+        limit: int,
+        exclude_ids: list[uuid.UUID],
+    ) -> tuple[str, list[RadioPick]]:
+        exploration = min(1.0, max(0.0, exploration))
+
+        if self._recommender.is_available():
+            ids = await self._recommender.radio_track_ids(
+                seed_track_id=seed_track_id,
+                exploration=exploration,
+                limit=limit,
+                exclude_ids=exclude_ids,
+            )
+            if ids is not None:
+                picks = [
+                    RadioPick(track=t, reason=REASON_ML)
+                    for t in await self._hydrate_tracks(ids)
+                ]
+                return REASON_ML, picks
+
+        return "metadata", await self._radio_fallback(
+            user_id=user_id,
+            seed_track_id=seed_track_id,
+            from_likes=from_likes,
+            exploration=exploration,
+            limit=limit,
+            exclude_ids=exclude_ids,
+        )
+
+    async def _radio_fallback(
+        self,
+        *,
+        user_id: uuid.UUID,
+        seed_track_id: uuid.UUID | None,
+        from_likes: bool,
+        exploration: float,
+        limit: int,
+        exclude_ids: list[uuid.UUID],
+    ) -> list[RadioPick]:
+        exclude = list(dict.fromkeys(exclude_ids))  # de-dupe, keep order
+        explore_n = round(limit * exploration)
+        similar_n = limit - explore_n
+        picks: list[RadioPick] = []
+
+        seed, seed_reason = await self._resolve_seed(
+            user_id, seed_track_id, from_likes
+        )
+        if seed is not None and similar_n > 0:
+            for track in await self._tracks.list_similar(
+                genre=seed.genre,
+                artist_id=seed.artist_id,
+                exclude_ids=exclude,
+                limit=similar_n,
+            ):
+                picks.append(RadioPick(track=track, reason=seed_reason))
+                exclude.append(track.id)
+
+        # Fill the remainder (exploration + any similarity shortfall) with random
+        # playable tracks — this is also the total fallback when there's no seed.
+        remaining = limit - len(picks)
+        if remaining > 0:
+            for track in await self._tracks.sample_playable(
+                exclude_ids=exclude, limit=remaining
+            ):
+                picks.append(RadioPick(track=track, reason=REASON_DISCOVER))
+                exclude.append(track.id)
+
+        random.shuffle(picks)
+        return picks
+
+    async def _resolve_seed(
+        self,
+        user_id: uuid.UUID,
+        seed_track_id: uuid.UUID | None,
+        from_likes: bool,
+    ) -> tuple[Track | None, str]:
+        if seed_track_id is not None:
+            return await self._tracks.get_by_id(seed_track_id), REASON_SIMILAR
+        if from_likes:
+            liked = await self._likes.list_liked_tracks(
+                user_id=user_id, limit=_LIKED_SEED_POOL, offset=0
+            )
+            if liked:
+                return random.choice(liked), REASON_FROM_LIKES
+        return None, REASON_DISCOVER
+
+    async def _hydrate_tracks(self, ids: list[uuid.UUID]) -> list[Track]:
+        """Resolve ids → tracks preserving order, skipping any that vanished.
+        One batched query rather than N per-id round-trips."""
+        by_id = {t.id: t for t in await self._tracks.get_many(ids)}
+        return [by_id[i] for i in ids if i in by_id]

app/application/sync_service.py

@@ -0,0 +1,140 @@
+"""Offline-first sync use cases: delta pull + idempotent push.
+
+The cursor is a server-clock timestamp obtained from the DB (``now``), so it is
+immune to app/DB clock skew. A pull returns everything a user changed in the
+half-open window ``(since, cursor]``; a push appends the event-log entries a
+client accumulated offline. Events carry a client-generated id, so a replay is a
+no-op (append is ``ON CONFLICT DO NOTHING``). Events for tracks this server does
+not have are skipped rather than rejected (graceful degradation).
+
+Known limitation (v1): the delta is unpaginated and, being wall-clock based, a
+write committing right on the cursor boundary under concurrency can slip a cycle
+— a periodic full resync (``since=None``) heals it.
+"""
+
+import datetime as dt
+import uuid
+from collections.abc import Awaitable, Callable
+from dataclasses import dataclass
+
+from app.domain.entities.history import PlayHistoryEntry
+from app.domain.entities.like import Like
+from app.domain.entities.playlist import Playlist
+from app.domain.entities.track import Track
+from app.domain.ports import (
+    HistoryRepository,
+    LikeRepository,
+    PlaylistRepository,
+    TrackRepository,
+)
+
+
+@dataclass(frozen=True, slots=True)
+class LikeEvent:
+    id: uuid.UUID
+    track_id: uuid.UUID
+    value: str
+    created_at: dt.datetime
+
+
+@dataclass(frozen=True, slots=True)
+class PlayEvent:
+    id: uuid.UUID
+    track_id: uuid.UUID
+    played_at: dt.datetime
+    play_duration_seconds: int | None
+    completed: bool
+
+
+@dataclass(frozen=True, slots=True)
+class PlaylistDelta:
+    playlist: Playlist
+    track_ids: list[uuid.UUID]
+
+
+@dataclass(frozen=True, slots=True)
+class SyncChanges:
+    cursor: dt.datetime
+    likes: list[Like]
+    plays: list[PlayHistoryEntry]
+    playlists: list[PlaylistDelta]
+    tracks: list[Track]
+
+
+@dataclass(frozen=True, slots=True)
+class SyncPushResult:
+    cursor: dt.datetime
+    accepted_likes: int
+    accepted_plays: int
+
+
+class SyncService:
+    def __init__(
+        self,
+        *,
+        likes: LikeRepository,
+        history: HistoryRepository,
+        playlists: PlaylistRepository,
+        tracks: TrackRepository,
+        now: Callable[[], Awaitable[dt.datetime]],
+    ) -> None:
+        self._likes = likes
+        self._history = history
+        self._playlists = playlists
+        self._tracks = tracks
+        self._now = now
+
+    async def get_changes(self, user_id: uuid.UUID, *, since: dt.datetime | None) -> SyncChanges:
+        until = await self._now()
+        likes = await self._likes.list_since(user_id, since=since, until=until)
+        plays = await self._history.list_since(user_id, since=since, until=until)
+        changed = await self._playlists.list_changed_since(
+            owner_id=user_id, since=since, until=until
+        )
+        playlists = [
+            PlaylistDelta(playlist=p, track_ids=await self._playlists.track_ids(p.id))
+            for p in changed
+        ]
+        tracks = await self._tracks.list_changed_since(since=since, until=until)
+        return SyncChanges(
+            cursor=until, likes=likes, plays=plays, playlists=playlists, tracks=tracks
+        )
+
+    async def push(
+        self,
+        user_id: uuid.UUID,
+        *,
+        likes: list[LikeEvent],
+        plays: list[PlayEvent],
+    ) -> SyncPushResult:
+        accepted_likes = 0
+        for event in likes:
+            if await self._tracks.get_by_id(event.track_id) is None:
+                continue  # skip events for tracks this server doesn't have
+            if await self._likes.add_event(
+                id=event.id,
+                user_id=user_id,
+                track_id=event.track_id,
+                value=event.value,
+                created_at=event.created_at,
+            ):
+                accepted_likes += 1
+
+        accepted_plays = 0
+        for play in plays:
+            if await self._tracks.get_by_id(play.track_id) is None:
+                continue
+            if await self._history.add_event(
+                id=play.id,
+                user_id=user_id,
+                track_id=play.track_id,
+                played_at=play.played_at,
+                play_duration_seconds=play.play_duration_seconds,
+                completed=play.completed,
+            ):
+                accepted_plays += 1
+
+        cursor = await self._now()
+        return SyncPushResult(
+            cursor=cursor, accepted_likes=accepted_likes, accepted_plays=accepted_plays
+        )

app/application/transcode_service.py

@@ -0,0 +1,110 @@
+"""Transcode service + cache-path helpers (Group B / plan §6.6).
+
+Cache layout under ``transcode_cache_path``::
+
+    {track_id}/opus_{kbps}.opus     # direct quality renditions
+    {track_id}/hls/playlist.m3u8    # HLS rendition (AAC-in-TS)
+    {track_id}/hls/seg_000.ts …
+
+The request side (streaming router) only *reads* the cache — misses fall back to
+the original file and enqueue generation. The worker (``transcode_task``) writes
+it. Path helpers are module-level so both sides agree on locations without one
+importing the other.
+"""
+
+import re
+import shutil
+import uuid
+from pathlib import Path
+
+import anyio
+
+from app.domain.errors import NotFoundError
+from app.domain.ports import TrackRepository
+
+# Stream-quality name (matches the user-settings ``StreamQuality``) → Opus
+# bitrate. ``original`` is absent: it means "serve the master, no transcode".
+QUALITY_BITRATE: dict[str, int] = {"high": 128, "medium": 96, "low": 64}
+
+# Single HLS rendition bitrate (AAC). One rendition keeps the MVP simple; a
+# multi-bitrate ladder can come later.
+HLS_BITRATE = 128
+
+# Only these segment names may be served, guarding the segment route against
+# path traversal.
+_SEGMENT_RE = re.compile(r"^seg_\d{3,}\.ts$")
+
+
+def bitrate_for_quality(quality: str) -> int | None:
+    """Opus bitrate for a quality name, or ``None`` for ``original``/unknown."""
+    return QUALITY_BITRATE.get(quality)
+
+
+def track_cache_dir(root: Path, track_id: uuid.UUID) -> Path:
+    return root / str(track_id)
+
+
+def opus_path(root: Path, track_id: uuid.UUID, bitrate_kbps: int) -> Path:
+    return track_cache_dir(root, track_id) / f"opus_{bitrate_kbps}.opus"
+
+
+def hls_dir(root: Path, track_id: uuid.UUID) -> Path:
+    return track_cache_dir(root, track_id) / "hls"
+
+
+def hls_playlist_path(root: Path, track_id: uuid.UUID) -> Path:
+    return hls_dir(root, track_id) / "playlist.m3u8"
+
+
+def hls_segment_path(root: Path, track_id: uuid.UUID, name: str) -> Path | None:
+    """Resolve a segment file, or ``None`` if the name is not a valid segment."""
+    if not _SEGMENT_RE.fullmatch(name):
+        return None
+    return hls_dir(root, track_id) / name
+
+
+def remove_track_cache(root: Path, track_id: uuid.UUID) -> None:
+    """Delete every cached rendition for a track (Opus + HLS). Best-effort — used
+    when a track is deleted so its transcode cache doesn't dangle forever."""
+    shutil.rmtree(track_cache_dir(root, track_id), ignore_errors=True)
+
+
+class TranscodeService:
+    """Request-side cache lookups for transcoded renditions."""
+
+    def __init__(self, *, tracks: TrackRepository, cache_root: Path) -> None:
+        self._tracks = tracks
+        self._root = cache_root
+
+    async def _require_streamable(self, track_id: uuid.UUID) -> None:
+        track = await self._tracks.get_by_id(track_id)
+        if track is None:
+            raise NotFoundError("Track not found.")
+        if track.storage_uri is None:
+            raise NotFoundError("Track is not yet downloaded.")
+
+    async def resolve_quality_file(
+        self, track_id: uuid.UUID, quality: str
+    ) -> Path | None:
+        """Cached Opus file for ``quality`` if present, else ``None`` (caller
+        falls back to the master and enqueues generation). ``original`` → None."""
+        bitrate = bitrate_for_quality(quality)
+        if bitrate is None:
+            return None
+        path = opus_path(self._root, track_id, bitrate)
+        exists = await anyio.to_thread.run_sync(path.exists)
+        return path if exists else None
+
+    async def hls_playlist(self, track_id: uuid.UUID) -> Path | None:
+        """Cached HLS playlist if generated, else ``None`` (validates the track
+        exists so an unknown id 404s rather than silently missing)."""
+        await self._require_streamable(track_id)
+        path = hls_playlist_path(self._root, track_id)
+        exists = await anyio.to_thread.run_sync(path.exists)
+        return path if exists else None
+
+    def hls_segment(self, track_id: uuid.UUID, name: str) -> Path | None:
+        path = hls_segment_path(self._root, track_id, name)
+        if path is None or not path.exists():
+            return None
+        return path

app/core/config.py

@@ -81,6 +81,9 @@ class Settings(BaseSettings):
     # -- media / storage --------------------------------------------------
     media_path: Path = Path("/data/media")
     transcode_cache_path: Path = Path("/data/transcode-cache")
+    # ffmpeg binary for transcoding/HLS (on PATH in the image); override for a
+    # non-standard location.
+    ffmpeg_path: str = "ffmpeg"
     max_parallel_downloads: int = 2
     # How many times the download worker retries a failed fetch (yt-dlp fails
     # often) before marking the job ``failed`` — exponential backoff between tries.

app/domain/entities/lyrics.py

@@ -0,0 +1,41 @@
+"""Lyrics value objects (plan §6.7).
+
+``Lyrics`` is the cached row for a track; ``LyricsResult`` is what a provider
+(LRCLIB) returns for a lookup. Both cross the domain boundary — no framework
+imports. Status values mirror ``LyricsStatus`` in the ORM enum ("found" /
+"not_found" / "pending") but are kept as plain strings here so the domain stays
+independent of the persistence layer.
+"""
+
+import datetime as dt
+import uuid
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True, slots=True)
+class LyricsResult:
+    """A provider hit: synced (timestamped LRC) and/or plain text."""
+
+    synced: str | None
+    plain: str | None
+    source: str
+
+
+@dataclass(frozen=True, slots=True)
+class Lyrics:
+    """Cached lyrics for one track. ``status`` is ``found`` / ``not_found`` /
+    ``pending``; ``not_found`` is cached too (with a TTL in the service) so a
+    track with no lyrics doesn't hammer the provider on every play."""
+
+    track_id: uuid.UUID
+    synced: str | None
+    plain: str | None
+    source: str | None
+    status: str
+    fetched_at: dt.datetime
+
+    @property
+    def has_lyrics(self) -> bool:
+        return self.status == "found" and (
+            self.synced is not None or self.plain is not None
+        )

app/domain/errors.py

@@ -76,6 +76,13 @@ class StorageError(DomainError):
     code = "storage_error"
 
 
+class TranscodeError(DomainError):
+    """Transcoding (ffmpeg) failed. Raised in the worker; a play falls back to
+    the original file rather than surfacing this."""
+
+    code = "transcode_error"
+
+
 class RangeNotSatisfiableError(DomainError):
     """Requested byte range cannot be satisfied."""
 

app/domain/ports.py

@@ -29,6 +29,7 @@ from app.domain.entities import (
     SubsonicCredentials,
     User,
 )
+from app.domain.entities.lyrics import Lyrics, LyricsResult
 from app.domain.entities.settings import UserSettings
 from app.domain.entities.track import Artist, Track
 from app.domain.sources import DownloadResult, RawMetadata, SearchResult, SourceFile, SourceInfo
@@ -127,6 +128,12 @@ class ArtistRepository(Protocol):
 
     async def get_by_id(self, artist_id: uuid.UUID) -> Artist | None: ...
     async def get_many(self, ids: list[uuid.UUID]) -> list[Artist]: ...
+    async def list_similar(self, *, artist_id: uuid.UUID, limit: int) -> list[Artist]:
+        """Artists sharing the seed artist's genres, ranked by overlap. Metadata
+        fallback for ``GET /artists/{id}/similar``. Defined before ``list`` so the
+        ``list[Artist]`` annotation isn't shadowed by the method named ``list``."""
+        ...
+
     async def list(self, *, q: str | None, limit: int, offset: int) -> list[Artist]: ...
     async def count(self, *, q: str | None) -> int: ...
     async def album_count(self, artist_id: uuid.UUID) -> int: ...
@@ -135,6 +142,11 @@ class ArtistRepository(Protocol):
 
 class TrackRepository(Protocol):
     async def get_by_id(self, track_id: uuid.UUID) -> Track | None: ...
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Track]:
+        """Resolve multiple ids in one query (unordered) — batches the per-id
+        lookups radio/similar would otherwise fan out into N round-trips."""
+        ...
+
     async def get_by_source(self, source: str, source_id: str) -> Track | None: ...
     async def add(
         self,
@@ -170,12 +182,33 @@ class TrackRepository(Protocol):
     # AlbumRepository below).
     async def genres(self) -> list[tuple[str, int]]: ...
     async def library_stats(self) -> LibraryStats: ...
+    async def list_similar(
+        self,
+        *,
+        genre: str | None,
+        artist_id: uuid.UUID,
+        exclude_ids: list[uuid.UUID],
+        limit: int,
+    ) -> list[Track]:
+        """Playable tracks resembling a seed (same genre and/or artist), ranked
+        by match strength then shuffled. The metadata fallback for §6.5 radio /
+        similar when no ML service is configured."""
+        ...
+
+    async def sample_playable(
+        self, *, exclude_ids: list[uuid.UUID], limit: int
+    ) -> list[Track]:
+        """Random playable tracks — the exploration filler for radio."""
+        ...
     async def find_duplicate_groups(self) -> list[tuple[str, list[Track]]]: ...
     async def list_by_metadata_status(
         self, status: str, *, limit: int, offset: int
     ) -> list[Track]: ...
     async def all_storage_refs(self) -> list[tuple[uuid.UUID, str]]: ...
     async def count_by_metadata_status(self, status: str) -> int: ...
+    async def list_changed_since(
+        self, *, since: dt.datetime | None, until: dt.datetime
+    ) -> list[Track]: ...
     async def list(
         self,
         *,
@@ -300,12 +333,28 @@ class PlaylistRepository(Protocol):
         self, playlist_id: uuid.UUID, ordered_track_ids: list[uuid.UUID]
     ) -> None: ...
     async def get_cover_path(self, playlist_id: uuid.UUID) -> str | None: ...
+    async def list_changed_since(
+        self, *, owner_id: uuid.UUID, since: dt.datetime | None, until: dt.datetime
+    ) -> list[Playlist]: ...
+    async def track_ids(self, playlist_id: uuid.UUID) -> list[uuid.UUID]: ...
     # list must come after any method using list[...] in its signature (name shadowing)
     async def list(self, *, owner_id: uuid.UUID, limit: int, offset: int) -> list[Playlist]: ...
 
 
 class LikeRepository(Protocol):
     async def add(self, *, user_id: uuid.UUID, track_id: uuid.UUID, value: str) -> Like: ...
+    async def add_event(
+        self,
+        *,
+        id: uuid.UUID,
+        user_id: uuid.UUID,
+        track_id: uuid.UUID,
+        value: str,
+        created_at: dt.datetime,
+    ) -> bool: ...
+    async def list_since(
+        self, user_id: uuid.UUID, *, since: dt.datetime | None, until: dt.datetime
+    ) -> list[Like]: ...
     async def get_latest_state(
         self, *, user_id: uuid.UUID, track_ids: list[uuid.UUID]
     ) -> list[Like]: ...
@@ -325,6 +374,19 @@ class HistoryRepository(Protocol):
         play_duration_seconds: int | None,
         completed: bool,
     ) -> PlayHistoryEntry: ...
+    async def add_event(
+        self,
+        *,
+        id: uuid.UUID,
+        user_id: uuid.UUID,
+        track_id: uuid.UUID,
+        played_at: dt.datetime,
+        play_duration_seconds: int | None,
+        completed: bool,
+    ) -> bool: ...
+    async def list_since(
+        self, user_id: uuid.UUID, *, since: dt.datetime | None, until: dt.datetime
+    ) -> list[PlayHistoryEntry]: ...
     async def list(
         self, *, user_id: uuid.UUID, limit: int, offset: int
     ) -> list[PlayHistoryEntry]: ...
@@ -462,3 +524,68 @@ class CoverArtProvider(Protocol):
 
     def is_available(self) -> bool: ...
     async def fetch_release_group(self, release_group_mbid: str) -> CoverArt | None: ...
+
+
+class Recommender(Protocol):
+    """External ML recommender (plan §6.5, ``ML_SERVICE_URL``). Returns ordered
+    track/artist ids, or ``None`` when unavailable/erroring so the service falls
+    back to metadata heuristics — ML is never a hard dependency (invariant)."""
+
+    def is_available(self) -> bool: ...
+
+    async def similar_track_ids(
+        self, track_id: uuid.UUID, *, limit: int, exclude_ids: list[uuid.UUID]
+    ) -> list[uuid.UUID] | None: ...
+
+    async def similar_artist_ids(
+        self, artist_id: uuid.UUID, *, limit: int
+    ) -> list[uuid.UUID] | None: ...
+
+    async def radio_track_ids(
+        self,
+        *,
+        seed_track_id: uuid.UUID | None,
+        exploration: float,
+        limit: int,
+        exclude_ids: list[uuid.UUID],
+    ) -> list[uuid.UUID] | None: ...
+
+
+class Transcoder(Protocol):
+    """Transcodes an audio file with ffmpeg (plan §6.6 / Group B). ``to_opus``
+    writes a single Opus rendition; ``to_hls`` writes an HLS playlist + segments
+    (AAC-in-TS) into ``out_dir``. Both raise ``TranscodeError`` on failure; heavy
+    work always runs in a worker, never the request cycle."""
+
+    async def to_opus(self, src: Path, dest: Path, *, bitrate_kbps: int) -> None: ...
+    async def to_hls(self, src: Path, out_dir: Path, *, bitrate_kbps: int) -> None: ...
+
+
+class LyricsProvider(Protocol):
+    """Fetches lyrics from an external database (LRCLIB) by artist/title/album/
+    duration. Returns a hit or ``None`` (no match / service down), never raising."""
+
+    async def fetch(
+        self,
+        *,
+        artist: str,
+        title: str,
+        album: str | None,
+        duration_seconds: int | None,
+    ) -> LyricsResult | None: ...
+
+
+class LyricsRepository(Protocol):
+    """Cached lyrics, one row per track. ``upsert`` also caches a ``not_found``
+    (empty text) so misses aren't re-fetched until the service's TTL lapses."""
+
+    async def get(self, track_id: uuid.UUID) -> Lyrics | None: ...
+    async def upsert(
+        self,
+        *,
+        track_id: uuid.UUID,
+        synced: str | None,
+        plain: str | None,
+        source: str | None,
+        status: str,
+    ) -> Lyrics: ...

app/infrastructure/db/models/like.py

@@ -37,3 +37,12 @@ class LikeModel(UUIDPrimaryKeyMixin, Base):
         server_default=func.now(),
         nullable=False,
     )
+    # Server ingestion time — the delta-sync ordering key. Set once at insert and
+    # never changed; distinct from ``created_at`` (the real event time, which a
+    # sync push preserves from the client even when it happened offline earlier).
+    synced_at: Mapped[dt.datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False,
+        index=True,
+    )

app/infrastructure/db/models/play_history.py

@@ -34,3 +34,11 @@ class PlayHistoryModel(UUIDPrimaryKeyMixin, Base):
     )
     play_duration_seconds: Mapped[int | None] = mapped_column(Integer, nullable=True)
     completed: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    # Server ingestion time — the delta-sync ordering key (see LikeModel). Distinct
+    # from ``played_at`` (the real play time), which a sync push preserves.
+    synced_at: Mapped[dt.datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False,
+        index=True,
+    )

app/infrastructure/db/repositories/__init__.py

@@ -7,6 +7,7 @@ from app.infrastructure.db.repositories.download_job_repository import (
 )
 from app.infrastructure.db.repositories.history_repository import SqlAlchemyHistoryRepository
 from app.infrastructure.db.repositories.like_repository import SqlAlchemyLikeRepository
+from app.infrastructure.db.repositories.lyrics_repository import SqlAlchemyLyricsRepository
 from app.infrastructure.db.repositories.playlist_repository import SqlAlchemyPlaylistRepository
 from app.infrastructure.db.repositories.refresh_token_repository import (
     SqlAlchemyRefreshTokenRepository,
@@ -23,6 +24,7 @@ __all__ = [
     "SqlAlchemyDownloadJobRepository",
     "SqlAlchemyHistoryRepository",
     "SqlAlchemyLikeRepository",
+    "SqlAlchemyLyricsRepository",
     "SqlAlchemyPlaylistRepository",
     "SqlAlchemyRefreshTokenRepository",
     "SqlAlchemyTrackRepository",

app/infrastructure/db/repositories/artist_repository.py

@@ -77,6 +77,26 @@ class SqlAlchemyArtistRepository:
         )
         return [_to_entity(r) for r in rows]
 
+    async def list_similar(self, *, artist_id: uuid.UUID, limit: int) -> list[Artist]:
+        # Artists whose tracks fall in the seed artist's genres, ranked by how
+        # many such tracks they have. Defined before ``list`` so the ``list[Artist]``
+        # return annotation isn't shadowed by the method named ``list``.
+        seed_genres = (
+            select(TrackModel.genre)
+            .where(TrackModel.artist_id == artist_id, TrackModel.genre.is_not(None))
+            .distinct()
+        )
+        stmt = (
+            select(ArtistModel)
+            .join(TrackModel, TrackModel.artist_id == ArtistModel.id)
+            .where(TrackModel.genre.in_(seed_genres), ArtistModel.id != artist_id)
+            .group_by(ArtistModel.id)
+            .order_by(func.count(TrackModel.id).desc())
+            .limit(limit)
+        )
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
     async def list(self, *, q: str | None, limit: int, offset: int) -> list[Artist]:
         stmt = select(ArtistModel)
         if q:
@@ -108,3 +128,4 @@ class SqlAlchemyArtistRepository:
                 .where(TrackModel.artist_id == artist_id)
             )
         ).scalar_one()
+

app/infrastructure/db/repositories/history_repository.py

@@ -4,6 +4,7 @@ import datetime as dt
 import uuid
 
 from sqlalchemy import func, select
+from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.domain.entities.history import PlayHistoryEntry
@@ -46,6 +47,51 @@ class SqlAlchemyHistoryRepository:
         await self._session.refresh(row)
         return _to_entity(row)
 
+    async def add_event(
+        self,
+        *,
+        id: uuid.UUID,
+        user_id: uuid.UUID,
+        track_id: uuid.UUID,
+        played_at: dt.datetime,
+        play_duration_seconds: int | None,
+        completed: bool,
+    ) -> bool:
+        """Idempotent append for sync push: insert a client-generated play event,
+        skipping it if the ``id`` already exists (a replay). Returns whether a new
+        row was stored. Defined before ``list`` (name-shadowing)."""
+        stmt = (
+            pg_insert(PlayHistoryModel)
+            .values(
+                id=id,
+                user_id=user_id,
+                track_id=track_id,
+                played_at=played_at,
+                play_duration_seconds=play_duration_seconds,
+                completed=completed,
+            )
+            .on_conflict_do_nothing(index_elements=["id"])
+            .returning(PlayHistoryModel.id)
+        )
+        inserted = (await self._session.execute(stmt)).scalar_one_or_none()
+        return inserted is not None
+
+    async def list_since(
+        self, user_id: uuid.UUID, *, since: dt.datetime | None, until: dt.datetime
+    ) -> list[PlayHistoryEntry]:
+        """Play events for a user ingested in the half-open window ``(since,
+        until]`` (by ``synced_at``, the server-side sync key — so events pushed
+        with an older ``played_at`` still surface), oldest first. ``since=None``
+        returns everything up to ``until``."""
+        stmt = select(PlayHistoryModel).where(
+            PlayHistoryModel.user_id == user_id, PlayHistoryModel.synced_at <= until
+        )
+        if since is not None:
+            stmt = stmt.where(PlayHistoryModel.synced_at > since)
+        stmt = stmt.order_by(PlayHistoryModel.synced_at)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
     async def list(self, *, user_id: uuid.UUID, limit: int, offset: int) -> list[PlayHistoryEntry]:
         rows = (
             (

app/infrastructure/db/repositories/like_repository.py

@@ -3,9 +3,11 @@
 Likes are an append-only event log. Current state = latest event per (user, track).
 """
 
+import datetime as dt
 import uuid
 
-from sqlalchemy import func, select
+from sqlalchemy import Subquery, func, select
+from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.domain.entities.like import Like
@@ -59,31 +61,96 @@ class SqlAlchemyLikeRepository:
         await self._session.refresh(row)
         return _to_entity(row)
 
+    async def add_event(
+        self,
+        *,
+        id: uuid.UUID,
+        user_id: uuid.UUID,
+        track_id: uuid.UUID,
+        value: str,
+        created_at: dt.datetime,
+    ) -> bool:
+        """Idempotent append for sync push: insert a client-generated like event,
+        skipping it if the ``id`` already exists (a replay). Preserves the
+        client's ``created_at`` (the event happened offline earlier). Returns
+        whether a new row was stored."""
+        stmt = (
+            pg_insert(LikeModel)
+            .values(
+                id=id,
+                user_id=user_id,
+                track_id=track_id,
+                value=value,
+                created_at=created_at,
+            )
+            .on_conflict_do_nothing(index_elements=["id"])
+            .returning(LikeModel.id)
+        )
+        inserted = (await self._session.execute(stmt)).scalar_one_or_none()
+        return inserted is not None
+
+    async def list_since(
+        self, user_id: uuid.UUID, *, since: dt.datetime | None, until: dt.datetime
+    ) -> list[Like]:
+        """Like events for a user ingested in the half-open window ``(since,
+        until]`` (by ``synced_at``, the server-side sync key — so events pushed
+        with an older ``created_at`` still surface), oldest first. ``since=None``
+        returns everything up to ``until``."""
+        stmt = select(LikeModel).where(
+            LikeModel.user_id == user_id, LikeModel.synced_at <= until
+        )
+        if since is not None:
+            stmt = stmt.where(LikeModel.synced_at > since)
+        stmt = stmt.order_by(LikeModel.synced_at)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
+    def _latest_events_sq(
+        self, user_id: uuid.UUID, track_ids: list[uuid.UUID] | None
+    ) -> Subquery:
+        """The latest like event per ``track_id`` for a user, as a subquery.
+
+        ``DISTINCT ON (track_id)`` with a deterministic tiebreaker (``created_at``
+        then ``id``) picks exactly one row per track even when two events share an
+        identical ``created_at`` — likes carry a client-supplied timestamp from
+        offline sync, so ties are realistic and a plain ``max()``+equality-join
+        would return both rows (double-counting the track)."""
+        stmt = select(
+            LikeModel.track_id,
+            LikeModel.value.label("value"),
+            LikeModel.created_at.label("created_at"),
+        ).where(LikeModel.user_id == user_id)
+        if track_ids is not None:
+            stmt = stmt.where(LikeModel.track_id.in_(track_ids))
+        return (
+            stmt.distinct(LikeModel.track_id)
+            .order_by(
+                LikeModel.track_id,
+                LikeModel.created_at.desc(),
+                LikeModel.id.desc(),
+            )
+            .subquery()
+        )
+
     async def get_latest_state(
         self, *, user_id: uuid.UUID, track_ids: list[uuid.UUID]
     ) -> list[Like]:
         if not track_ids:
             return []
-        # Subquery: max(created_at) per track for this user
-        max_sq = (
-            select(
-                LikeModel.track_id,
-                func.max(LikeModel.created_at).label("latest"),
-            )
-            .where(LikeModel.user_id == user_id, LikeModel.track_id.in_(track_ids))
-            .group_by(LikeModel.track_id)
-            .subquery()
-        )
         rows = (
             (
                 await self._session.execute(
                     select(LikeModel)
-                    .join(
-                        max_sq,
-                        (LikeModel.track_id == max_sq.c.track_id)
-                        & (LikeModel.created_at == max_sq.c.latest),
+                    .where(
+                        LikeModel.user_id == user_id,
+                        LikeModel.track_id.in_(track_ids),
+                    )
+                    .distinct(LikeModel.track_id)
+                    .order_by(
+                        LikeModel.track_id,
+                        LikeModel.created_at.desc(),
+                        LikeModel.id.desc(),
                     )
-                    .where(LikeModel.user_id == user_id)
                 )
             )
             .scalars()
@@ -95,31 +162,14 @@ class SqlAlchemyLikeRepository:
         self, *, user_id: uuid.UUID, limit: int, offset: int
     ) -> list[Track]:
         # Tracks where the latest like event has value='like', ordered by like time desc
-        max_sq = (
-            select(
-                LikeModel.track_id,
-                func.max(LikeModel.created_at).label("latest"),
-            )
-            .where(LikeModel.user_id == user_id)
-            .group_by(LikeModel.track_id)
-            .subquery()
-        )
-        liked_sq = (
-            select(LikeModel.track_id, LikeModel.created_at)
-            .join(
-                max_sq,
-                (LikeModel.track_id == max_sq.c.track_id)
-                & (LikeModel.created_at == max_sq.c.latest),
-            )
-            .where(LikeModel.user_id == user_id, LikeModel.value == "like")
-            .subquery()
-        )
+        latest_sq = self._latest_events_sq(user_id, None)
         rows = (
             (
                 await self._session.execute(
                     select(TrackModel)
-                    .join(liked_sq, TrackModel.id == liked_sq.c.track_id)
-                    .order_by(liked_sq.c.created_at.desc())
+                    .join(latest_sq, TrackModel.id == latest_sq.c.track_id)
+                    .where(latest_sq.c.value == "like")
+                    .order_by(latest_sq.c.created_at.desc())
                     .limit(limit)
                     .offset(offset)
                 )
@@ -130,25 +180,11 @@ class SqlAlchemyLikeRepository:
         return [_track_to_entity(r) for r in rows]
 
     async def count_liked_tracks(self, *, user_id: uuid.UUID) -> int:
-        max_sq = (
-            select(
-                LikeModel.track_id,
-                func.max(LikeModel.created_at).label("latest"),
-            )
-            .where(LikeModel.user_id == user_id)
-            .group_by(LikeModel.track_id)
-            .subquery()
-        )
-        liked_sq = (
-            select(LikeModel.track_id)
-            .join(
-                max_sq,
-                (LikeModel.track_id == max_sq.c.track_id)
-                & (LikeModel.created_at == max_sq.c.latest),
-            )
-            .where(LikeModel.user_id == user_id, LikeModel.value == "like")
-            .subquery()
-        )
+        latest_sq = self._latest_events_sq(user_id, None)
         return (
-            await self._session.execute(select(func.count()).select_from(liked_sq))
+            await self._session.execute(
+                select(func.count())
+                .select_from(latest_sq)
+                .where(latest_sq.c.value == "like")
+            )
         ).scalar_one()

app/infrastructure/db/repositories/lyrics_repository.py

@@ -0,0 +1,72 @@
+"""Lyrics repository — adapter over ``AsyncSession``.
+
+One cached row per track (``track_id`` unique). ``upsert`` refreshes the row and
+bumps ``fetched_at`` so the service's TTL is measured from the last fetch.
+"""
+
+import uuid
+
+from sqlalchemy import func, select
+from sqlalchemy.dialects.postgresql import insert as pg_insert
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.domain.entities.lyrics import Lyrics
+from app.infrastructure.db.models.lyrics import LyricsModel
+
+
+def _to_entity(row: LyricsModel) -> Lyrics:
+    return Lyrics(
+        track_id=row.track_id,
+        synced=row.synced,
+        plain=row.plain,
+        source=row.source,
+        status=row.status,
+        fetched_at=row.fetched_at,
+    )
+
+
+class SqlAlchemyLyricsRepository:
+    def __init__(self, session: AsyncSession) -> None:
+        self._session = session
+
+    async def get(self, track_id: uuid.UUID) -> Lyrics | None:
+        row = await self._session.scalar(
+            select(LyricsModel).where(LyricsModel.track_id == track_id)
+        )
+        return _to_entity(row) if row is not None else None
+
+    async def upsert(
+        self,
+        *,
+        track_id: uuid.UUID,
+        synced: str | None,
+        plain: str | None,
+        source: str | None,
+        status: str,
+    ) -> Lyrics:
+        values = {
+            "track_id": track_id,
+            "synced": synced,
+            "plain": plain,
+            "source": source,
+            "status": status,
+            "fetched_at": func.now(),
+        }
+        stmt = (
+            pg_insert(LyricsModel)
+            .values(**values)
+            .on_conflict_do_update(
+                index_elements=[LyricsModel.track_id],
+                set_={
+                    "synced": synced,
+                    "plain": plain,
+                    "source": source,
+                    "status": status,
+                    "fetched_at": func.now(),
+                },
+            )
+            .returning(LyricsModel)
+        )
+        row = (await self._session.scalars(stmt)).one()
+        await self._session.flush()
+        return _to_entity(row)

app/infrastructure/db/repositories/playlist_repository.py

@@ -1,5 +1,6 @@
 """Playlist repository — adapter over ``AsyncSession``."""
 
+import datetime as dt
 import uuid
 
 from sqlalchemy import func, select
@@ -216,6 +217,32 @@ class SqlAlchemyPlaylistRepository:
             )
         ).scalar_one_or_none()
 
+    async def list_changed_since(
+        self, *, owner_id: uuid.UUID, since: dt.datetime | None, until: dt.datetime
+    ) -> list[Playlist]:
+        """A user's playlists changed in the window ``(since, until]`` (metadata
+        or membership — every mutation bumps ``updated_at``/``version``). Defined
+        before ``list`` (name-shadowing)."""
+        stmt = select(PlaylistModel).where(
+            PlaylistModel.owner_id == owner_id, PlaylistModel.updated_at <= until
+        )
+        if since is not None:
+            stmt = stmt.where(PlaylistModel.updated_at > since)
+        stmt = stmt.order_by(PlaylistModel.updated_at)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
+    async def track_ids(self, playlist_id: uuid.UUID) -> list[uuid.UUID]:
+        """Ordered track ids of a playlist (by position) — for sync payloads."""
+        rows = (
+            await self._session.execute(
+                select(PlaylistTrackModel.track_id)
+                .where(PlaylistTrackModel.playlist_id == playlist_id)
+                .order_by(PlaylistTrackModel.position)
+            )
+        ).scalars().all()
+        return list(rows)
+
     # list must come after methods using list[...] in signatures (builtin name shadowing)
     async def list(self, *, owner_id: uuid.UUID, limit: int, offset: int) -> list[Playlist]:
         rows = (

app/infrastructure/db/repositories/track_repository.py

@@ -3,7 +3,7 @@
 import datetime as dt
 import uuid
 
-from sqlalchemy import func, select
+from sqlalchemy import case, func, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.domain.entities.storage import FormatBreakdown, LibraryStats
@@ -46,6 +46,16 @@ class SqlAlchemyTrackRepository:
         row = await self._session.get(TrackModel, track_id)
         return _to_entity(row) if row is not None else None
 
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Track]:
+        if not ids:
+            return []
+        rows = (
+            (await self._session.execute(select(TrackModel).where(TrackModel.id.in_(ids))))
+            .scalars()
+            .all()
+        )
+        return [_to_entity(r) for r in rows]
+
     async def get_by_source(self, source: str, source_id: str) -> Track | None:
         row = (
             await self._session.execute(
@@ -136,6 +146,42 @@ class SqlAlchemyTrackRepository:
         ).all()
         return [(row.genre, row.cnt) for row in rows]
 
+    async def list_similar(
+        self,
+        *,
+        genre: str | None,
+        artist_id: uuid.UUID,
+        exclude_ids: list[uuid.UUID],
+        limit: int,
+    ) -> list[Track]:
+        # Rank a same-genre hit above a same-artist hit; shuffle within a tier so
+        # the mix varies. Only playable (locally-stored) tracks are candidates.
+        if genre is not None:
+            match = or_(TrackModel.genre == genre, TrackModel.artist_id == artist_id)
+            score = case((TrackModel.genre == genre, 2), else_=0) + case(
+                (TrackModel.artist_id == artist_id, 1), else_=0
+            )
+        else:
+            match = TrackModel.artist_id == artist_id
+            score = case((TrackModel.artist_id == artist_id, 1), else_=0)
+
+        stmt = select(TrackModel).where(TrackModel.storage_uri.is_not(None), match)
+        if exclude_ids:
+            stmt = stmt.where(TrackModel.id.not_in(exclude_ids))
+        stmt = stmt.order_by(score.desc(), func.random()).limit(limit)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
+    async def sample_playable(
+        self, *, exclude_ids: list[uuid.UUID], limit: int
+    ) -> list[Track]:
+        stmt = select(TrackModel).where(TrackModel.storage_uri.is_not(None))
+        if exclude_ids:
+            stmt = stmt.where(TrackModel.id.not_in(exclude_ids))
+        stmt = stmt.order_by(func.random()).limit(limit)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
     async def library_stats(self) -> LibraryStats:
         """One-shot aggregate over the whole catalogue (no pagination). Defined
         before ``list`` for the same shadowing reason as ``genres``."""
@@ -266,6 +312,19 @@ class SqlAlchemyTrackRepository:
             )
         ).scalar_one()
 
+    async def list_changed_since(
+        self, *, since: dt.datetime | None, until: dt.datetime
+    ) -> list[Track]:
+        """Catalogue tracks changed in the window ``(since, until]`` (by
+        ``updated_at``), oldest first — the delta a client caches for offline use.
+        Defined before ``list`` (name-shadowing)."""
+        stmt = select(TrackModel).where(TrackModel.updated_at <= until)
+        if since is not None:
+            stmt = stmt.where(TrackModel.updated_at > since)
+        stmt = stmt.order_by(TrackModel.updated_at)
+        rows = (await self._session.execute(stmt)).scalars().all()
+        return [_to_entity(r) for r in rows]
+
     async def list(
         self,
         *,

app/infrastructure/metadata/lrclib.py

@@ -0,0 +1,100 @@
+"""LrclibHttpClient — fetches lyrics from LRCLIB (plan §6.7).
+
+LRCLIB is a free, keyless lyrics database. ``/api/get`` does an exact match on
+artist+track+album+duration; if that misses we fall back to ``/api/search`` and
+take the best-scoring hit. Graceful degradation: any network/parse error →
+``fetch`` returns ``None`` (the service then caches a ``not_found``), never
+raising. No API key is needed, so this provider is always "available".
+"""
+
+import httpx
+
+from app.core.logging import get_logger
+from app.domain.entities.lyrics import LyricsResult
+
+log = get_logger(__name__)
+
+_BASE_URL = "https://lrclib.net"
+_TIMEOUT_SECONDS = 10.0
+_SOURCE = "lrclib"
+
+
+class LrclibHttpClient:
+    """Implements :class:`app.domain.ports.LyricsProvider`."""
+
+    def __init__(self, *, user_agent: str, base_url: str = _BASE_URL) -> None:
+        self._user_agent = user_agent
+        self._base_url = base_url.rstrip("/")
+
+    async def fetch(
+        self,
+        *,
+        artist: str,
+        title: str,
+        album: str | None,
+        duration_seconds: int | None,
+    ) -> LyricsResult | None:
+        try:
+            async with httpx.AsyncClient(
+                timeout=_TIMEOUT_SECONDS,
+                headers={"User-Agent": self._user_agent},
+                base_url=self._base_url,
+            ) as client:
+                hit = await self._get(client, artist, title, album, duration_seconds)
+                if hit is None:
+                    hit = await self._search(client, artist, title)
+        except (httpx.HTTPError, ValueError) as exc:
+            log.warning("lrclib.fetch_failed", error=str(exc))
+            return None
+        return hit
+
+    async def _get(
+        self,
+        client: httpx.AsyncClient,
+        artist: str,
+        title: str,
+        album: str | None,
+        duration_seconds: int | None,
+    ) -> LyricsResult | None:
+        """Exact match via ``/api/get`` (404 when nothing matches exactly)."""
+        params = {"artist_name": artist, "track_name": title}
+        if album:
+            params["album_name"] = album
+        if duration_seconds is not None:
+            params["duration"] = str(duration_seconds)
+        resp = await client.get("/api/get", params=params)
+        if resp.status_code == httpx.codes.NOT_FOUND:
+            return None
+        resp.raise_for_status()
+        return _to_result(resp.json())
+
+    async def _search(
+        self, client: httpx.AsyncClient, artist: str, title: str
+    ) -> LyricsResult | None:
+        """Fuzzy fallback via ``/api/search`` — take the first usable hit."""
+        resp = await client.get(
+            "/api/search", params={"artist_name": artist, "track_name": title}
+        )
+        resp.raise_for_status()
+        results = resp.json()
+        if not isinstance(results, list):
+            return None
+        for item in results:
+            result = _to_result(item)
+            if result is not None:
+                return result
+        return None
+
+
+def _to_result(payload: object) -> LyricsResult | None:
+    """Map an LRCLIB record to a ``LyricsResult``. Instrumental tracks and empty
+    records yield ``None`` (nothing worth caching as "found")."""
+    if not isinstance(payload, dict):
+        return None
+    if payload.get("instrumental"):
+        return None
+    synced = payload.get("syncedLyrics") or None
+    plain = payload.get("plainLyrics") or None
+    if synced is None and plain is None:
+        return None
+    return LyricsResult(synced=synced, plain=plain, source=_SOURCE)

app/infrastructure/ml/__init__.py

@@ -0,0 +1 @@
+"""ML/recommender adapters (plan §6.5). ML is optional — see Recommender port."""

app/infrastructure/ml/recommender.py

@@ -0,0 +1,110 @@
+"""Recommender adapters (plan §6.5).
+
+``NullRecommender`` is the default: no ML service, so every method reports
+unavailable and the ``RecommendationService`` uses its metadata fallback. When
+an embedding service exists, wire ``RemoteRecommender`` (skeleton below) to
+``ML_SERVICE_URL`` — its exact request/response contract is TODO pending that
+service. Both keep the invariant: ML is optional, never a hard dependency.
+"""
+
+import uuid
+
+import httpx
+
+from app.core.logging import get_logger
+
+log = get_logger(__name__)
+
+
+class NullRecommender:
+    """No ML configured — always unavailable, always ``None`` (→ fallback)."""
+
+    def is_available(self) -> bool:
+        return False
+
+    async def similar_track_ids(
+        self, track_id: uuid.UUID, *, limit: int, exclude_ids: list[uuid.UUID]
+    ) -> list[uuid.UUID] | None:
+        return None
+
+    async def similar_artist_ids(
+        self, artist_id: uuid.UUID, *, limit: int
+    ) -> list[uuid.UUID] | None:
+        return None
+
+    async def radio_track_ids(
+        self,
+        *,
+        seed_track_id: uuid.UUID | None,
+        exploration: float,
+        limit: int,
+        exclude_ids: list[uuid.UUID],
+    ) -> list[uuid.UUID] | None:
+        return None
+
+
+_TIMEOUT_SECONDS = 5.0
+
+
+class RemoteRecommender:
+    """HTTP client for an external embedding/recommender service.
+
+    TODO: the request/response schema below is a placeholder — align it with the
+    real ML service once its contract is known. Until then this stays unused
+    (``deps`` wires ``NullRecommender``). Every call is defensive: any error or a
+    malformed body returns ``None`` so the service degrades to metadata, matching
+    the graceful-degradation invariant.
+    """
+
+    def __init__(self, base_url: str) -> None:
+        self._base_url = base_url.rstrip("/")
+
+    def is_available(self) -> bool:
+        return True
+
+    async def _post_ids(self, path: str, payload: dict[str, object]) -> list[uuid.UUID] | None:
+        try:
+            async with httpx.AsyncClient(timeout=_TIMEOUT_SECONDS) as client:
+                resp = await client.post(f"{self._base_url}{path}", json=payload)
+                resp.raise_for_status()
+                data = resp.json()
+            ids = data.get("track_ids") if isinstance(data, dict) else None
+            if not isinstance(ids, list):
+                return None
+            return [uuid.UUID(str(i)) for i in ids]
+        except (httpx.HTTPError, ValueError, KeyError) as exc:
+            log.warning("recommender.remote_failed", path=path, error=str(exc))
+            return None
+
+    async def similar_track_ids(
+        self, track_id: uuid.UUID, *, limit: int, exclude_ids: list[uuid.UUID]
+    ) -> list[uuid.UUID] | None:
+        return await self._post_ids(
+            "/similar/tracks",
+            {"track_id": str(track_id), "limit": limit,
+             "exclude": [str(i) for i in exclude_ids]},
+        )
+
+    async def similar_artist_ids(
+        self, artist_id: uuid.UUID, *, limit: int
+    ) -> list[uuid.UUID] | None:
+        # Artist recommendations aren't part of the placeholder track contract.
+        return None
+
+    async def radio_track_ids(
+        self,
+        *,
+        seed_track_id: uuid.UUID | None,
+        exploration: float,
+        limit: int,
+        exclude_ids: list[uuid.UUID],
+    ) -> list[uuid.UUID] | None:
+        return await self._post_ids(
+            "/radio",
+            {
+                "seed_track_id": str(seed_track_id) if seed_track_id else None,
+                "exploration": exploration,
+                "limit": limit,
+                "exclude": [str(i) for i in exclude_ids],
+            },
+        )

app/infrastructure/transcode/__init__.py

@@ -0,0 +1 @@
+"""ffmpeg-based transcoding adapters (Group B / plan §6.6)."""

app/infrastructure/transcode/ffmpeg.py

@@ -0,0 +1,71 @@
+"""FfmpegTranscoder — Opus + HLS renditions via the ffmpeg CLI.
+
+Implements :class:`app.domain.ports.Transcoder`. Runs ffmpeg as a subprocess
+(only ever from a worker — CLAUDE.md: no heavy work in the request cycle) and
+raises :class:`TranscodeError` on a non-zero exit. Opus is used for the direct
+quality renditions; HLS segments are AAC-in-MPEG-TS (broad player support).
+"""
+
+import asyncio
+import uuid
+from pathlib import Path
+
+import anyio
+
+from app.core.logging import get_logger
+from app.domain.errors import TranscodeError
+
+log = get_logger(__name__)
+
+# HLS segment length. 10s is a common VOD default — few requests, quick seeks.
+_HLS_SEGMENT_SECONDS = "10"
+
+
+def _mkdir(path: Path) -> None:
+    path.mkdir(parents=True, exist_ok=True)
+
+
+class FfmpegTranscoder:
+    def __init__(self, ffmpeg_path: str = "ffmpeg") -> None:
+        self._ffmpeg = ffmpeg_path
+
+    async def to_opus(self, src: Path, dest: Path, *, bitrate_kbps: int) -> None:
+        await anyio.to_thread.run_sync(_mkdir, dest.parent)
+        # Per-writer temp name (not a shared ``.part``) so two concurrent jobs for
+        # the same rendition can't interleave into one file — each writes its own
+        # temp and the last atomic replace wins, both leaving a valid output.
+        tmp = dest.with_name(f"{dest.name}.{uuid.uuid4().hex}.part")
+        await self._run(
+            "-i", str(src),
+            "-vn", "-c:a", "libopus", "-b:a", f"{bitrate_kbps}k",
+            "-f", "opus", str(tmp),
+        )
+        # Publish atomically so a concurrent reader never sees a half-written file.
+        await anyio.to_thread.run_sync(tmp.replace, dest)
+
+    async def to_hls(self, src: Path, out_dir: Path, *, bitrate_kbps: int) -> None:
+        await anyio.to_thread.run_sync(_mkdir, out_dir)
+        playlist = out_dir / "playlist.m3u8"
+        segments = out_dir / "seg_%03d.ts"
+        await self._run(
+            "-i", str(src),
+            "-vn", "-c:a", "aac", "-b:a", f"{bitrate_kbps}k",
+            "-f", "hls",
+            "-hls_time", _HLS_SEGMENT_SECONDS,
+            "-hls_playlist_type", "vod",
+            "-hls_segment_filename", str(segments),
+            str(playlist),
+        )
+
+    async def _run(self, *args: str) -> None:
+        cmd = [self._ffmpeg, "-y", "-nostdin", "-loglevel", "error", *args]
+        proc = await asyncio.create_subprocess_exec(
+            *cmd,
+            stdout=asyncio.subprocess.DEVNULL,
+            stderr=asyncio.subprocess.PIPE,
+        )
+        _, stderr = await proc.communicate()
+        if proc.returncode != 0:
+            detail = stderr.decode(errors="replace").strip()[-500:]
+            log.error("ffmpeg_failed", returncode=proc.returncode, error=detail)
+            raise TranscodeError(f"ffmpeg exited {proc.returncode}: {detail}")

app/workers/arq_worker.py

@@ -14,6 +14,7 @@ from app.workers.tasks.download_task import download_track
 from app.workers.tasks.enrich_task import enrich_track
 from app.workers.tasks.import_task import scan_local_folder
 from app.workers.tasks.materialize_task import materialize_track
+from app.workers.tasks.transcode_task import transcode_track
 
 log = get_logger("worker")
 
@@ -35,6 +36,7 @@ class WorkerSettings:
         download_track,
         materialize_track,
         cleanup_storage,
+        transcode_track,
     ]
     on_startup = startup
     on_shutdown = shutdown

app/workers/queue.py

@@ -59,6 +59,28 @@ async def enqueue_materialize(job_id: uuid.UUID) -> None:
         log.warning("materialize_enqueue_failed", job_id=str(job_id))
 
 
+async def enqueue_transcode(
+    track_id: uuid.UUID, *, quality: str = "high", hls: bool = True
+) -> str:
+    """Enqueue a transcode job (§6.6). Unlike the best-effort follow-ups below,
+    this is user/stream-driven, so it surfaces a job id (and a 503 via the caller
+    if the queue is down) rather than swallowing failures."""
+    return await enqueue(
+        "transcode_track", track_id=str(track_id), quality=quality, hls=hls
+    )
+
+
+async def enqueue_transcode_quiet(
+    track_id: uuid.UUID, *, quality: str = "high", hls: bool = False
+) -> None:
+    """Best-effort transcode enqueue for a streaming cache miss — never blocks or
+    fails the play (the master is served meanwhile)."""
+    try:
+        await enqueue_transcode(track_id, quality=quality, hls=hls)
+    except DependencyUnavailableError:
+        log.warning("transcode_enqueue_failed", track_id=str(track_id))
+
+
 async def enqueue_enrich(track_id: uuid.UUID) -> None:
     """Best-effort enqueue of metadata enrichment for a freshly stored track.
 

app/workers/tasks/transcode_task.py

@@ -0,0 +1,82 @@
+"""arq task: transcode a track to a cached Opus rendition and/or HLS (§6.6).
+
+Triggered by ``POST /tracks/{id}/optimize`` and by a streaming cache miss. Reads
+the master once (via ``storage.as_local_path``) and writes into
+``transcode_cache_path``. Idempotent — existing outputs are skipped, so repeated
+enqueues (e.g. several plays before the first finishes) are cheap. The DB session
+is released before ffmpeg runs; the track entity is a detached value object.
+"""
+
+import shutil
+import uuid
+from pathlib import Path
+from typing import Any
+
+import anyio
+
+from app.application.transcode_service import (
+    HLS_BITRATE,
+    bitrate_for_quality,
+    hls_dir,
+    hls_playlist_path,
+    opus_path,
+)
+from app.core.config import get_settings
+from app.core.logging import get_logger
+from app.infrastructure.db import session_scope
+from app.infrastructure.db.repositories import SqlAlchemyTrackRepository
+from app.infrastructure.storage.provider import get_file_storage
+from app.infrastructure.transcode.ffmpeg import FfmpegTranscoder
+
+log = get_logger("worker.transcode")
+
+
+def _swap_dir(tmp_dir: Path, final_dir: Path) -> None:
+    """Publish a freshly-built HLS rendition atomically: replace the final dir in
+    one rename so a reader never sees a playlist referencing half-written
+    segments. Any stale partial at the destination is cleared first."""
+    if final_dir.exists():
+        shutil.rmtree(final_dir)
+    tmp_dir.replace(final_dir)
+
+
+async def transcode_track(
+    _ctx: dict[str, Any],
+    *,
+    track_id: str,
+    quality: str = "high",
+    hls: bool = True,
+) -> dict[str, Any]:
+    settings = get_settings()
+    tid = uuid.UUID(track_id)
+
+    async with session_scope() as session:
+        track = await SqlAlchemyTrackRepository(session).get_by_id(tid)
+    if track is None or track.storage_uri is None:
+        log.warning("transcode_skip_no_file", track_id=track_id)
+        return {"track_id": track_id, "opus": False, "hls": False}
+
+    storage = get_file_storage()
+    transcoder = FfmpegTranscoder(settings.ffmpeg_path)
+    root = settings.transcode_cache_path
+    did_opus = False
+    did_hls = False
+
+    async with storage.as_local_path(track.storage_uri) as src:
+        bitrate = bitrate_for_quality(quality)
+        if bitrate is not None:
+            dest = opus_path(root, tid, bitrate)
+            if not dest.exists():
+                await transcoder.to_opus(src, dest, bitrate_kbps=bitrate)
+                did_opus = True
+        if hls and not hls_playlist_path(root, tid).exists():
+            # Build into a private temp dir, then swap it in atomically — the
+            # cached playlist only becomes visible once every segment is written.
+            final_dir = hls_dir(root, tid)
+            tmp_dir = final_dir.with_name(f"hls.{uuid.uuid4().hex}.tmp")
+            await transcoder.to_hls(src, tmp_dir, bitrate_kbps=HLS_BITRATE)
+            await anyio.to_thread.run_sync(_swap_dir, tmp_dir, final_dir)
+            did_hls = True
+
+    log.info("transcode_done", track_id=track_id, opus=did_opus, hls=did_hls)
+    return {"track_id": track_id, "opus": did_opus, "hls": did_hls}

tests/test_admin_api.py

@@ -128,7 +128,9 @@ async def test_settings_exposes_effective_config_without_secrets(api: AsyncClien
     resp = await api.get("/api/v1/admin/settings", headers=headers)
     assert resp.status_code == 200, resp.text
     body = resp.json()
-    assert body["environment"] == "test"
+    # environment reflects however the process booted (test on host, dev in the
+    # container) — assert it's a valid value, not a specific one.
+    assert body["environment"] in ("dev", "test", "prod")
     assert body["storage_backend"] == "local"
     assert body["allow_registration"] is True
     # No secret material should ever appear in the payload.

tests/test_lyrics.py

@@ -0,0 +1,247 @@
+"""Lyrics service (§6.7) + OpenSubsonic structured-lyrics serializer — DB-free.
+
+Service: get-or-fetch caching, not_found TTL, force refetch, graceful miss.
+Serializer: LRC → timed lines, plain fallback, empty payload when absent.
+"""
+
+import datetime as dt
+import uuid
+
+import pytest
+from app.api.rest.serializers import _parse_lrc, structured_lyrics
+from app.application.lyrics_service import LyricsService
+from app.domain.entities import Artist, Track
+from app.domain.entities.album import Album
+from app.domain.entities.lyrics import Lyrics, LyricsResult
+from app.domain.errors import NotFoundError
+
+
+def _now() -> dt.datetime:
+    return dt.datetime.now(dt.UTC)
+
+
+def _lyrics(*, synced=None, plain=None, source=None, status="found", age_days=0) -> Lyrics:
+    return Lyrics(
+        track_id=uuid.uuid4(),
+        synced=synced,
+        plain=plain,
+        source=source,
+        status=status,
+        fetched_at=_now() - dt.timedelta(days=age_days),
+    )
+
+
+def _track() -> Track:
+    now = _now()
+    return Track(
+        id=uuid.uuid4(),
+        title="Song",
+        artist_id=uuid.uuid4(),
+        album_id=None,
+        storage_uri="tracks/x.mp3",
+        file_format="mp3",
+        file_size=1,
+        source="upload",
+        source_id="x",
+        duration_seconds=200,
+        genre=None,
+        year=None,
+        track_number=None,
+        metadata_status="pending",
+        metadata_error=None,
+        enriched_at=None,
+        availability="local",
+        created_at=now,
+        updated_at=now,
+    )
+
+
+class FakeLyricsRepo:
+    def __init__(self, cached: Lyrics | None) -> None:
+        self._cached = cached
+        self.upserts: list[dict[str, object]] = []
+
+    async def get(self, track_id: uuid.UUID) -> Lyrics | None:
+        return self._cached
+
+    async def upsert(self, *, track_id, synced, plain, source, status) -> Lyrics:
+        self.upserts.append({"status": status, "source": source})
+        return Lyrics(
+            track_id=track_id,
+            synced=synced,
+            plain=plain,
+            source=source,
+            status=status,
+            fetched_at=_now(),
+        )
+
+
+class FakeTrackRepo:
+    def __init__(self, track: Track | None) -> None:
+        self._track = track
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._track
+
+
+class FakeArtistRepo:
+    def __init__(self, artist: Artist | None = None) -> None:
+        self._artist = artist
+
+    async def get_by_id(self, artist_id: uuid.UUID) -> Artist | None:
+        return self._artist
+
+
+class FakeAlbumRepo:
+    async def get_by_id(self, album_id: uuid.UUID) -> Album | None:
+        return None
+
+
+class FakeProvider:
+    def __init__(self, result: LyricsResult | None) -> None:
+        self._result = result
+        self.calls = 0
+
+    async def fetch(self, *, artist, title, album, duration_seconds) -> LyricsResult | None:
+        self.calls += 1
+        return self._result
+
+
+def _service(*, cached, track, provider) -> tuple[LyricsService, FakeLyricsRepo, FakeProvider]:
+    repo = FakeLyricsRepo(cached)
+    prov = FakeProvider(provider)
+    svc = LyricsService(
+        lyrics=repo,
+        tracks=FakeTrackRepo(track),
+        artists=FakeArtistRepo(),
+        albums=FakeAlbumRepo(),
+        provider=prov,
+    )
+    return svc, repo, prov
+
+
+# -- service ------------------------------------------------------------------
+
+
+async def test_fresh_found_is_served_from_cache_without_provider() -> None:
+    cached = _lyrics(plain="hi", status="found")
+    svc, _repo, prov = _service(cached=cached, track=_track(), provider=None)
+    out = await svc.get_lyrics(cached.track_id)
+    assert out is cached
+    assert prov.calls == 0
+
+
+async def test_miss_fetches_and_caches_found() -> None:
+    track = _track()
+    result = LyricsResult(synced="[00:01.00]hi", plain="hi", source="lrclib")
+    svc, repo, prov = _service(cached=None, track=track, provider=result)
+    out = await svc.get_lyrics(track.id)
+    assert prov.calls == 1
+    assert out.status == "found"
+    assert repo.upserts[-1]["status"] == "found"
+
+
+async def test_provider_miss_caches_not_found() -> None:
+    track = _track()
+    svc, repo, prov = _service(cached=None, track=track, provider=None)
+    out = await svc.get_lyrics(track.id)
+    assert prov.calls == 1
+    assert out.status == "not_found"
+    assert repo.upserts[-1]["status"] == "not_found"
+
+
+async def test_stale_not_found_is_refetched() -> None:
+    # A not_found older than the 7-day TTL is retried against the provider.
+    stale = _lyrics(status="not_found", age_days=8)
+    result = LyricsResult(synced=None, plain="found now", source="lrclib")
+    svc, _repo, prov = _service(cached=stale, track=_track(), provider=result)
+    out = await svc.get_lyrics(stale.track_id)
+    assert prov.calls == 1
+    assert out.status == "found"
+
+
+async def test_fresh_not_found_is_not_refetched() -> None:
+    fresh = _lyrics(status="not_found", age_days=1)
+    svc, _repo, prov = _service(cached=fresh, track=_track(), provider=None)
+    out = await svc.get_lyrics(fresh.track_id)
+    assert out is fresh
+    assert prov.calls == 0
+
+
+async def test_force_bypasses_a_fresh_cache() -> None:
+    cached = _lyrics(plain="old", status="found")
+    result = LyricsResult(synced=None, plain="new", source="lrclib")
+    svc, _repo, prov = _service(cached=cached, track=_track(), provider=result)
+    out = await svc.get_lyrics(cached.track_id, force=True)
+    assert prov.calls == 1
+    assert out.plain == "new"
+
+
+async def test_unknown_track_raises() -> None:
+    svc, _repo, _prov = _service(cached=None, track=None, provider=None)
+    with pytest.raises(NotFoundError):
+        await svc.get_lyrics(uuid.uuid4())
+
+
+# -- serializer ---------------------------------------------------------------
+
+
+def test_parse_lrc_times_and_order() -> None:
+    lines = _parse_lrc("[00:12.34]second\n[00:01.00]first\n[bad]meta\n\n")
+    # Metadata/blank lines dropped; output ordered by time; centiseconds → ms.
+    assert lines == [
+        {"start": 1000, "value": "first"},
+        {"start": 12340, "value": "second"},
+    ]
+
+
+def test_parse_lrc_repeated_stamps_on_one_line() -> None:
+    lines = _parse_lrc("[00:01.00][00:05.00]chorus")
+    assert lines == [
+        {"start": 1000, "value": "chorus"},
+        {"start": 5000, "value": "chorus"},
+    ]
+
+
+def test_structured_lyrics_prefers_synced() -> None:
+    out = structured_lyrics(
+        _lyrics(synced="[00:01.00]hi", plain="hi", status="found"),
+        display_artist="A",
+        display_title="T",
+    )
+    entry = out["lyricsList"]["structuredLyrics"][0]
+    assert entry["synced"] is True
+    assert entry["displayArtist"] == "A"
+    assert entry["line"] == [{"start": 1000, "value": "hi"}]
+
+
+def test_structured_lyrics_falls_back_to_plain() -> None:
+    out = structured_lyrics(
+        _lyrics(synced=None, plain="line one\nline two", status="found"),
+        display_artist="A",
+        display_title="T",
+    )
+    entry = out["lyricsList"]["structuredLyrics"][0]
+    assert entry["synced"] is False
+    assert entry["line"] == [{"value": "line one"}, {"value": "line two"}]
+
+
+def test_structured_lyrics_empty_when_absent() -> None:
+    out = structured_lyrics(
+        _lyrics(synced=None, plain=None, status="not_found"),
+        display_artist="A",
+        display_title="T",
+    )
+    assert out == {"lyricsList": {}}
+
+
+def test_structured_lyrics_unparseable_synced_falls_back_to_plain() -> None:
+    # synced present but no valid timecodes → treat as plain, synced=False.
+    out = structured_lyrics(
+        _lyrics(synced="no timecodes here", plain="plain text", status="found"),
+        display_artist="A",
+        display_title="T",
+    )
+    entry = out["lyricsList"]["structuredLyrics"][0]
+    assert entry["synced"] is False
+    assert entry["line"] == [{"value": "plain text"}]

tests/test_recommendation_service.py

@@ -0,0 +1,248 @@
+"""RecommendationService (§6.5) — DB-free, in-memory fakes.
+
+Covers the two paths that matter: the ML recommender when available (reason
+``ml``, order-preserving batched hydration) and the metadata fallback when it
+declines (similar/radio still work — the graceful-degradation invariant).
+"""
+
+import datetime as dt
+import uuid
+
+import pytest
+from app.application.recommendation_service import (
+    REASON_FROM_LIKES,
+    REASON_ML,
+    REASON_SIMILAR,
+    RecommendationService,
+)
+from app.domain.entities import Artist, Track
+from app.domain.errors import NotFoundError
+from app.infrastructure.ml.recommender import NullRecommender
+
+
+def _now() -> dt.datetime:
+    return dt.datetime.now(dt.UTC)
+
+
+def _track(*, genre: str | None = "rock", artist_id: uuid.UUID | None = None) -> Track:
+    now = _now()
+    return Track(
+        id=uuid.uuid4(),
+        title="T",
+        artist_id=artist_id or uuid.uuid4(),
+        album_id=None,
+        storage_uri="tracks/x.mp3",
+        file_format="mp3",
+        file_size=1,
+        source="upload",
+        source_id="x",
+        duration_seconds=None,
+        genre=genre,
+        year=None,
+        track_number=None,
+        metadata_status="pending",
+        metadata_error=None,
+        enriched_at=None,
+        availability="local",
+        created_at=now,
+        updated_at=now,
+    )
+
+
+def _artist() -> Artist:
+    now = _now()
+    return Artist(
+        id=uuid.uuid4(), name="A", source=None, source_id=None, created_at=now, updated_at=now
+    )
+
+
+class FakeTrackRepo:
+    def __init__(self, tracks: list[Track]) -> None:
+        self._by_id = {t.id: t for t in tracks}
+        self.similar: list[Track] = []
+        self.sample: list[Track] = []
+        self.similar_calls: list[dict[str, object]] = []
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._by_id.get(track_id)
+
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Track]:
+        # Deliberately unordered (mirrors a real ``WHERE id IN`` query) so the
+        # service is responsible for restoring request order.
+        return [self._by_id[i] for i in reversed(ids) if i in self._by_id]
+
+    async def list_similar(self, *, genre, artist_id, exclude_ids, limit) -> list[Track]:
+        self.similar_calls.append({"exclude_ids": list(exclude_ids), "limit": limit})
+        return [t for t in self.similar if t.id not in exclude_ids][:limit]
+
+    async def sample_playable(self, *, exclude_ids, limit) -> list[Track]:
+        return [t for t in self.sample if t.id not in exclude_ids][:limit]
+
+
+class FakeArtistRepo:
+    def __init__(self, artists: list[Artist]) -> None:
+        self._by_id = {a.id: a for a in artists}
+        self.similar: list[Artist] = []
+
+    async def get_by_id(self, artist_id: uuid.UUID) -> Artist | None:
+        return self._by_id.get(artist_id)
+
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Artist]:
+        return [self._by_id[i] for i in reversed(ids) if i in self._by_id]
+
+    async def list_similar(self, *, artist_id, limit) -> list[Artist]:
+        return self.similar[:limit]
+
+
+class FakeLikeRepo:
+    def __init__(self, liked: list[Track]) -> None:
+        self.liked = liked
+
+    async def list_liked_tracks(self, *, user_id, limit, offset) -> list[Track]:
+        return self.liked[offset : offset + limit]
+
+
+class StubRecommender:
+    """An 'available' ML recommender returning fixed ids for the ML path."""
+
+    def __init__(self, ids: list[uuid.UUID] | None) -> None:
+        self._ids = ids
+
+    def is_available(self) -> bool:
+        return True
+
+    async def similar_track_ids(self, track_id, *, limit, exclude_ids):
+        return self._ids
+
+    async def similar_artist_ids(self, artist_id, *, limit):
+        return self._ids
+
+    async def radio_track_ids(self, *, seed_track_id, exploration, limit, exclude_ids):
+        return self._ids
+
+
+def _service(tracks, artists, likes, recommender) -> RecommendationService:
+    return RecommendationService(
+        recommender=recommender, tracks=tracks, artists=artists, likes=likes
+    )
+
+
+# -- similar ------------------------------------------------------------------
+
+
+async def test_similar_tracks_unknown_seed_raises() -> None:
+    svc = _service(FakeTrackRepo([]), FakeArtistRepo([]), FakeLikeRepo([]), NullRecommender())
+    with pytest.raises(NotFoundError):
+        await svc.similar_tracks(uuid.uuid4(), limit=5)
+
+
+async def test_similar_tracks_falls_back_to_metadata() -> None:
+    seed = _track()
+    neighbours = [_track(), _track()]
+    repo = FakeTrackRepo([seed, *neighbours])
+    repo.similar = neighbours
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), NullRecommender())
+
+    reason, found = await svc.similar_tracks(seed.id, limit=5)
+    assert reason == REASON_SIMILAR
+    assert [t.id for t in found] == [n.id for n in neighbours]
+    # The seed itself is always excluded from its own neighbours.
+    assert repo.similar_calls[-1]["exclude_ids"] == [seed.id]
+
+
+async def test_similar_tracks_uses_ml_and_preserves_order() -> None:
+    seed = _track()
+    a, b, c = _track(), _track(), _track()
+    repo = FakeTrackRepo([seed, a, b, c])
+    # ML returns a specific order; hydration must preserve it despite get_many
+    # returning rows unordered, and skip ids that no longer exist.
+    ml_ids = [c.id, uuid.uuid4(), a.id, b.id]
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), StubRecommender(ml_ids))
+
+    reason, found = await svc.similar_tracks(seed.id, limit=10)
+    assert reason == REASON_ML
+    assert [t.id for t in found] == [c.id, a.id, b.id]
+
+
+async def test_similar_artists_falls_back_to_metadata() -> None:
+    seed = _artist()
+    neighbours = [_artist(), _artist()]
+    repo = FakeArtistRepo([seed])
+    repo.similar = neighbours
+    svc = _service(FakeTrackRepo([]), repo, FakeLikeRepo([]), NullRecommender())
+
+    reason, found = await svc.similar_artists(seed.id, limit=5)
+    assert reason == REASON_SIMILAR
+    assert [a.id for a in found] == [n.id for n in neighbours]
+
+
+# -- radio --------------------------------------------------------------------
+
+
+async def test_radio_from_likes_seeds_and_fills(monkeypatch) -> None:
+    liked = _track()
+    similar = [_track(), _track()]
+    explore = [_track(), _track(), _track()]
+    repo = FakeTrackRepo([liked, *similar, *explore])
+    repo.similar = similar
+    repo.sample = explore
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([liked]), NullRecommender())
+    # Deterministic seed choice + no shuffling for a stable assertion.
+    monkeypatch.setattr("app.application.recommendation_service.random.choice", lambda s: s[0])
+    monkeypatch.setattr("app.application.recommendation_service.random.shuffle", lambda s: None)
+
+    reason, picks = await svc.radio(
+        user_id=uuid.uuid4(),
+        seed_track_id=None,
+        from_likes=True,
+        exploration=0.5,
+        limit=4,
+        exclude_ids=[],
+    )
+    assert reason == "metadata"
+    ids = [p.track.id for p in picks]
+    # 4 picks, all distinct, no seed repeated, mix of similar + discover.
+    assert len(ids) == 4
+    assert len(set(ids)) == 4
+    reasons = {p.reason for p in picks}
+    assert REASON_FROM_LIKES in reasons  # similar picks carry the seed's reason
+
+
+async def test_radio_respects_exclude_ids(monkeypatch) -> None:
+    similar = [_track(), _track()]
+    explore = [_track(), _track()]
+    repo = FakeTrackRepo([*similar, *explore])
+    repo.similar = similar
+    repo.sample = explore
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), NullRecommender())
+    monkeypatch.setattr("app.application.recommendation_service.random.shuffle", lambda s: None)
+
+    excluded = similar[0].id
+    _, picks = await svc.radio(
+        user_id=uuid.uuid4(),
+        seed_track_id=None,
+        from_likes=False,
+        exploration=1.0,
+        limit=4,
+        exclude_ids=[excluded],
+    )
+    assert excluded not in {p.track.id for p in picks}
+
+
+async def test_radio_uses_ml_when_available(monkeypatch) -> None:
+    a, b = _track(), _track()
+    repo = FakeTrackRepo([a, b])
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), StubRecommender([b.id, a.id]))
+    monkeypatch.setattr("app.application.recommendation_service.random.shuffle", lambda s: None)
+
+    reason, picks = await svc.radio(
+        user_id=uuid.uuid4(),
+        seed_track_id=a.id,
+        from_likes=False,
+        exploration=0.3,
+        limit=5,
+        exclude_ids=[],
+    )
+    assert reason == REASON_ML
+    assert [p.track.id for p in picks] == [b.id, a.id]
+    assert all(p.reason == REASON_ML for p in picks)

tests/test_sources_api.py

@@ -59,6 +59,13 @@ async def api(tmp_path: Path) -> AsyncIterator[AsyncClient]:
     os.environ["LOCAL_MEDIA_IMPORT_PATH"] = str(music)
     get_settings.cache_clear()
 
+    # The source registry is process-cached (lru_cache); another test hitting a
+    # registry-backed endpoint (e.g. /admin/sources) may have built it before
+    # LOCAL_MEDIA_IMPORT_PATH was set. Clear it so this test sees a fresh one.
+    from app.api.deps import get_source_registry
+
+    get_source_registry.cache_clear()
+
     import app.infrastructure.storage.provider as _storage_provider
 
     _storage_provider._storage = None
@@ -94,6 +101,7 @@ async def api(tmp_path: Path) -> AsyncIterator[AsyncClient]:
         os.environ.pop("MEDIA_PATH", None)
         os.environ.pop("LOCAL_MEDIA_IMPORT_PATH", None)
         get_settings.cache_clear()
+        get_source_registry.cache_clear()
 
 
 async def _login(api: AsyncClient) -> str:

tests/test_sync_api.py

@@ -0,0 +1,252 @@
+"""Integration tests for the offline-first sync endpoints.
+
+Drives POST /sync/push (idempotent append of like/play events) and GET
+/sync/changes (delta pull with a server-clock cursor). Requires a reachable
+Postgres; skips otherwise.
+"""
+
+import asyncio
+import os
+import uuid
+from collections.abc import AsyncIterator
+from pathlib import Path
+
+import pytest
+from app.core.config import get_settings
+from app.infrastructure.db import Base, dispose_engine, get_engine, session_scope
+from app.infrastructure.db.repositories import (
+    SqlAlchemyArtistRepository,
+    SqlAlchemyRefreshTokenRepository,
+    SqlAlchemyTrackRepository,
+    SqlAlchemyUserRepository,
+)
+from asgi_lifespan import LifespanManager
+from httpx import ASGITransport, AsyncClient
+
+pytestmark = pytest.mark.asyncio
+
+_db_reachable_cache: bool | None = None
+
+
+async def _db_reachable() -> bool:
+    global _db_reachable_cache
+    if _db_reachable_cache is not None:
+        return _db_reachable_cache
+    from sqlalchemy import text
+
+    try:
+        async with asyncio.timeout(3):
+            async with get_engine().connect() as conn:
+                await conn.execute(text("SELECT 1"))
+        _db_reachable_cache = True
+    except Exception:
+        _db_reachable_cache = False
+    return _db_reachable_cache
+
+
+async def _seed_track(source_id: str = "sync-1") -> uuid.UUID:
+    async with session_scope() as session:
+        artist = await SqlAlchemyArtistRepository(session).get_or_create("Sync Artist")
+        tid = uuid.uuid4()
+        await SqlAlchemyTrackRepository(session).add(
+            id=tid,
+            title="Sync Track",
+            artist_id=artist.id,
+            storage_uri=f"tracks/zz/{source_id}.mp3",
+            file_format="mp3",
+            file_size=10,
+            source="upload",
+            source_id=source_id,
+            metadata_status="enriched",
+            added_by=None,
+        )
+        return tid
+
+
+@pytest.fixture
+async def api(tmp_path: Path) -> AsyncIterator[AsyncClient]:
+    if not await _db_reachable():
+        pytest.skip("Postgres not reachable — integration test skipped.")
+
+    os.environ["MEDIA_PATH"] = str(tmp_path)
+    get_settings.cache_clear()
+
+    try:
+        async with get_engine().begin() as conn:
+            await conn.run_sync(Base.metadata.drop_all)
+            await conn.run_sync(Base.metadata.create_all)
+
+        from app.application.user_service import UserService
+        from app.core.security import Argon2PasswordHasher
+
+        async with session_scope() as session:
+            await UserService(
+                users=SqlAlchemyUserRepository(session),
+                refresh_tokens=SqlAlchemyRefreshTokenRepository(session),
+                hasher=Argon2PasswordHasher(),
+            ).create_user(username="syncer", password="testpass1", is_superuser=False)
+
+        from app.main import create_app
+
+        app = create_app()
+        async with LifespanManager(app):
+            transport = ASGITransport(app=app)
+            async with AsyncClient(transport=transport, base_url="http://test") as client:
+                yield client
+
+        async with get_engine().begin() as conn:
+            await conn.run_sync(Base.metadata.drop_all)
+        await dispose_engine()
+    finally:
+        os.environ.pop("MEDIA_PATH", None)
+        get_settings.cache_clear()
+
+
+async def _auth(api: AsyncClient) -> dict[str, str]:
+    resp = await api.post(
+        "/api/v1/auth/login", json={"username": "syncer", "password": "testpass1"}
+    )
+    assert resp.status_code == 200, resp.text
+    return {"Authorization": f"Bearer {resp.json()['access_token']}"}
+
+
+async def test_push_is_idempotent_and_pull_returns_events(api: AsyncClient) -> None:
+    headers = await _auth(api)
+    track_id = await _seed_track()
+    like_id, play_id = str(uuid.uuid4()), str(uuid.uuid4())
+
+    push_body = {
+        "likes": [
+            {
+                "id": like_id,
+                "track_id": str(track_id),
+                "value": "like",
+                "created_at": "2020-01-01T00:00:00+00:00",
+            }
+        ],
+        "plays": [
+            {
+                "id": play_id,
+                "track_id": str(track_id),
+                "played_at": "2020-01-01T00:05:00+00:00",
+                "play_duration_seconds": 180,
+                "completed": True,
+            }
+        ],
+    }
+
+    first = await api.post("/api/v1/sync/push", json=push_body, headers=headers)
+    assert first.status_code == 200, first.text
+    assert first.json()["accepted_likes"] == 1
+    assert first.json()["accepted_plays"] == 1
+
+    # Replaying the exact same events must store nothing (idempotent by id).
+    replay = await api.post("/api/v1/sync/push", json=push_body, headers=headers)
+    assert replay.json()["accepted_likes"] == 0
+    assert replay.json()["accepted_plays"] == 0
+
+    # Full pull returns the events + the catalogue track, with a cursor.
+    changes = await api.get("/api/v1/sync/changes", headers=headers)
+    assert changes.status_code == 200, changes.text
+    body = changes.json()
+    assert [lk["id"] for lk in body["likes"]] == [like_id]
+    assert body["likes"][0]["created_at"].startswith("2020-01-01")  # event time preserved
+    assert [p["id"] for p in body["plays"]] == [play_id]
+    assert str(track_id) in [t["id"] for t in body["tracks"]]
+    assert body["cursor"]
+
+
+async def test_cursor_advances_and_empty_delta(api: AsyncClient) -> None:
+    headers = await _auth(api)
+    track_id = await _seed_track()
+    await api.post(
+        "/api/v1/sync/push",
+        json={
+            "likes": [
+                {
+                    "id": str(uuid.uuid4()),
+                    "track_id": str(track_id),
+                    "value": "like",
+                    "created_at": "2024-01-01T00:00:00+00:00",
+                }
+            ]
+        },
+        headers=headers,
+    )
+    cursor = (await api.get("/api/v1/sync/changes", headers=headers)).json()["cursor"]
+
+    # Nothing changed since the cursor → empty delta (but a fresh cursor).
+    delta = await api.get("/api/v1/sync/changes", params={"since": cursor}, headers=headers)
+    assert delta.status_code == 200, delta.text
+    assert delta.json()["likes"] == []
+    assert delta.json()["plays"] == []
+
+
+async def test_offline_event_with_old_time_still_syncs(api: AsyncClient) -> None:
+    # The delta is keyed by server-ingestion time, not the client event time, so
+    # an event pushed now with an old created_at still surfaces for a cursor that
+    # sits *after* that old event time.
+    headers = await _auth(api)
+    track_id = await _seed_track()
+    baseline = (await api.get("/api/v1/sync/changes", headers=headers)).json()["cursor"]
+
+    await api.post(
+        "/api/v1/sync/push",
+        json={
+            "likes": [
+                {
+                    "id": str(uuid.uuid4()),
+                    "track_id": str(track_id),
+                    "value": "like",
+                    "created_at": "2019-06-01T00:00:00+00:00",  # long before `baseline`
+                }
+            ]
+        },
+        headers=headers,
+    )
+
+    delta = await api.get("/api/v1/sync/changes", params={"since": baseline}, headers=headers)
+    assert len(delta.json()["likes"]) == 1  # surfaced despite the old event time
+
+
+async def test_push_skips_unknown_track(api: AsyncClient) -> None:
+    headers = await _auth(api)
+    resp = await api.post(
+        "/api/v1/sync/push",
+        json={
+            "likes": [
+                {
+                    "id": str(uuid.uuid4()),
+                    "track_id": str(uuid.uuid4()),  # not in the catalogue
+                    "value": "like",
+                    "created_at": "2024-01-01T00:00:00+00:00",
+                }
+            ]
+        },
+        headers=headers,
+    )
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["accepted_likes"] == 0
+
+
+async def test_pull_includes_changed_playlists_with_track_ids(api: AsyncClient) -> None:
+    headers = await _auth(api)
+    track_id = await _seed_track()
+
+    created = await api.post("/api/v1/playlists", json={"name": "Sync Mix"}, headers=headers)
+    playlist_id = created.json()["id"]
+    await api.post(
+        f"/api/v1/playlists/{playlist_id}/tracks",
+        json={"track_id": str(track_id)},
+        headers=headers,
+    )
+
+    body = (await api.get("/api/v1/sync/changes", headers=headers)).json()
+    playlists = {p["id"]: p for p in body["playlists"]}
+    assert playlist_id in playlists
+    assert playlists[playlist_id]["track_ids"] == [str(track_id)]
+
+
+async def test_sync_requires_auth(api: AsyncClient) -> None:
+    assert (await api.get("/api/v1/sync/changes")).status_code == 401
+    assert (await api.post("/api/v1/sync/push", json={})).status_code == 401

tests/test_transcode_service.py

@@ -0,0 +1,157 @@
+"""Transcode cache-path helpers + request-side service (§6.6) — DB-free.
+
+Covers the pure path math, the segment-name traversal guard, cache
+hit/miss lookups, and the "not yet downloaded" / unknown-track 404 paths.
+"""
+
+import datetime as dt
+import uuid
+from pathlib import Path
+
+import pytest
+from app.application.transcode_service import (
+    QUALITY_BITRATE,
+    TranscodeService,
+    bitrate_for_quality,
+    hls_playlist_path,
+    hls_segment_path,
+    opus_path,
+    remove_track_cache,
+    track_cache_dir,
+)
+from app.domain.entities import Track
+from app.domain.errors import NotFoundError
+
+
+def _track(*, storage_uri: str | None = "tracks/aa/song.mp3") -> Track:
+    now = dt.datetime.now(dt.UTC)
+    return Track(
+        id=uuid.uuid4(),
+        title="Song",
+        artist_id=uuid.uuid4(),
+        album_id=None,
+        storage_uri=storage_uri,
+        file_format="mp3",
+        file_size=1,
+        source="upload",
+        source_id="x",
+        duration_seconds=None,
+        genre=None,
+        year=None,
+        track_number=None,
+        metadata_status="pending",
+        metadata_error=None,
+        enriched_at=None,
+        availability="local",
+        created_at=now,
+        updated_at=now,
+    )
+
+
+class FakeTrackRepo:
+    def __init__(self, track: Track | None) -> None:
+        self._track = track
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._track
+
+
+# -- pure helpers -------------------------------------------------------------
+
+
+def test_bitrate_for_quality_known_and_unknown() -> None:
+    assert bitrate_for_quality("high") == QUALITY_BITRATE["high"]
+    assert bitrate_for_quality("medium") == 96
+    assert bitrate_for_quality("low") == 64
+    # "original" and anything unrecognised mean "serve the master, no transcode".
+    assert bitrate_for_quality("original") is None
+    assert bitrate_for_quality("nonsense") is None
+
+
+def test_path_helpers_are_under_the_track_dir(tmp_path: Path) -> None:
+    tid = uuid.uuid4()
+    cache = track_cache_dir(tmp_path, tid)
+    assert cache == tmp_path / str(tid)
+    assert opus_path(tmp_path, tid, 128) == cache / "opus_128.opus"
+    assert hls_playlist_path(tmp_path, tid) == cache / "hls" / "playlist.m3u8"
+
+
+@pytest.mark.parametrize(
+    "name",
+    ["seg_000.ts", "seg_1234.ts"],
+)
+def test_hls_segment_path_accepts_valid_names(tmp_path: Path, name: str) -> None:
+    tid = uuid.uuid4()
+    resolved = hls_segment_path(tmp_path, tid, name)
+    assert resolved == track_cache_dir(tmp_path, tid) / "hls" / name
+
+
+@pytest.mark.parametrize(
+    "name",
+    ["../../etc/passwd", "seg_.ts", "seg_00.tsx", "playlist.m3u8", "seg_00.ts", "..", ""],
+)
+def test_hls_segment_path_rejects_traversal_and_junk(tmp_path: Path, name: str) -> None:
+    assert hls_segment_path(tmp_path, uuid.uuid4(), name) is None
+
+
+def test_remove_track_cache_is_best_effort(tmp_path: Path) -> None:
+    tid = uuid.uuid4()
+    d = track_cache_dir(tmp_path, tid)
+    (d / "hls").mkdir(parents=True)
+    (d / "opus_128.opus").write_bytes(b"x")
+    remove_track_cache(tmp_path, tid)
+    assert not d.exists()
+    # Idempotent: deleting an already-absent cache doesn't raise.
+    remove_track_cache(tmp_path, tid)
+
+
+# -- request-side service -----------------------------------------------------
+
+
+async def test_resolve_quality_file_hit_and_miss(tmp_path: Path) -> None:
+    track = _track()
+    svc = TranscodeService(tracks=FakeTrackRepo(track), cache_root=tmp_path)
+
+    # Miss: nothing on disk yet.
+    assert await svc.resolve_quality_file(track.id, "high") is None
+    # "original" never has a rendition.
+    assert await svc.resolve_quality_file(track.id, "original") is None
+
+    # Hit: once the rendition exists it's returned.
+    path = opus_path(tmp_path, track.id, 128)
+    path.parent.mkdir(parents=True)
+    path.write_bytes(b"opus")
+    assert await svc.resolve_quality_file(track.id, "high") == path
+
+
+async def test_hls_playlist_requires_downloaded_track(tmp_path: Path) -> None:
+    tid = uuid.uuid4()
+    # Unknown track → 404.
+    with pytest.raises(NotFoundError):
+        await TranscodeService(tracks=FakeTrackRepo(None), cache_root=tmp_path).hls_playlist(tid)
+
+    # Known but not downloaded (no storage_uri) → 404.
+    pending = _track(storage_uri=None)
+    with pytest.raises(NotFoundError):
+        await TranscodeService(tracks=FakeTrackRepo(pending), cache_root=tmp_path).hls_playlist(
+            pending.id
+        )
+
+
+async def test_hls_playlist_miss_then_hit(tmp_path: Path) -> None:
+    track = _track()
+    svc = TranscodeService(tracks=FakeTrackRepo(track), cache_root=tmp_path)
+    assert await svc.hls_playlist(track.id) is None
+
+    playlist = hls_playlist_path(tmp_path, track.id)
+    playlist.parent.mkdir(parents=True)
+    playlist.write_text("#EXTM3U")
+    assert await svc.hls_playlist(track.id) == playlist
+
+
+def test_hls_segment_returns_none_for_missing_file(tmp_path: Path) -> None:
+    svc = TranscodeService(tracks=FakeTrackRepo(_track()), cache_root=tmp_path)
+    # Valid name but no file on disk.
+    assert svc.hls_segment(uuid.uuid4(), "seg_000.ts") is None
+    # Junk name is refused outright.
+    assert svc.hls_segment(uuid.uuid4(), "../secret") is None