feat(reco): radio + similar with metadata fallback (§6.5)

POST /radio + /radio/next (stateless infinite feed: seed track / from-likes,
exploration mix, client-passed exclude_ids) and GET /tracks|artists/{id}/similar,
replacing the stubs. Recommender port abstracts the (future) ML service —
NullRecommender is wired now so RecommendationService always uses its metadata
heuristics (genre/artist similarity, random exploration filler), never a hard ML
dependency. Adds TrackRepository.list_similar/sample_playable + Artist.list_similar,
reason codes for the client, RemoteRecommender skeleton (TODO: ML contract).

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

app/api/deps.py

@@ -20,6 +20,7 @@ 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
@@ -51,6 +52,7 @@ 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
@@ -188,6 +190,18 @@ def get_transcode_service(session: SessionDep) -> TranscodeService:
     )
 
 
+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``."""
@@ -226,6 +240,9 @@ 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)]
 

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/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/tracks.py

@@ -1,7 +1,7 @@
 """Track endpoints."""
 
 import uuid
-from typing import Annotated, Any
+from typing import Annotated
 
 from fastapi import APIRouter, Query, Response
 from fastapi.responses import StreamingResponse
@@ -14,6 +14,7 @@ from app.api.deps import (
     FileStorageDep,
     LyricsServiceDep,
     MetadataServiceDep,
+    RecommendationServiceDep,
     RemoteLibraryServiceDep,
     StreamUser,
     TrackRepoDep,
@@ -21,6 +22,7 @@ from app.api.deps import (
 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,
@@ -221,7 +223,23 @@ async def delete_track(
 
 
 @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", status_code=202)