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/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,
+    )