feat(api): offline-first sync layer

Implements the stubbed /sync endpoints:
- GET /sync/changes — delta pull (likes, plays, changed playlists with
  their track ids, changed catalogue tracks) over the half-open window
  (since, cursor]; the cursor is the DB clock, so it's immune to app/DB
  skew.
- POST /sync/push — idempotent append of client like/play events
  (ON CONFLICT DO NOTHING by client-supplied id); events for tracks the
  server doesn't have are skipped (graceful degradation).

Adds a server-ingestion column `synced_at` to the likes + play_history
event logs (migration) as the delta ordering key, so an event pushed
with an older event time still surfaces for other devices. Repos gain
list_since/add_event (likes, history), list_changed_since (playlists,
tracks) and playlist.track_ids; wired via SyncService in deps.

Also fixes test isolation exposed by the registry-backed /admin/sources
endpoint: test_sources_api clears the process-cached source registry,
and test_admin_api no longer hardcodes the environment name.

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

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
+        )