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/domain/ports.py

@@ -176,6 +176,9 @@ class TrackRepository(Protocol):
     ) -> 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 +303,28 @@ 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 +344,19 @@ 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]: ...