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/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 = (
             (