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/api/schemas/sync.py

@@ -0,0 +1,82 @@
+"""Offline-first sync schemas (delta pull + idempotent push).
+
+The client keeps an opaque ``cursor`` (a server-clock timestamp). It pulls
+everything changed in the half-open window ``(since, cursor]`` and pushes the
+append-only events it accumulated offline. Events carry a client-generated
+``id`` so a replay is idempotent.
+"""
+
+import datetime as dt
+import uuid
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+from app.api.schemas.track import TrackOut
+
+LikeValue = Literal["like", "dislike", "neutral"]
+
+
+# -- pull (server -> client) --------------------------------------------------
+class LikeEventOut(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    value: str
+    created_at: dt.datetime
+
+
+class PlayEventOut(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    played_at: dt.datetime
+    play_duration_seconds: int | None
+    completed: bool
+
+
+class PlaylistSyncOut(BaseModel):
+    id: uuid.UUID
+    name: str
+    description: str | None
+    version: int
+    updated_at: dt.datetime
+    track_ids: list[uuid.UUID]
+
+
+class SyncChangesOut(BaseModel):
+    """Everything that changed for the caller since their last cursor. Feed
+    ``cursor`` back as ``?since=`` on the next pull."""
+
+    cursor: dt.datetime
+    likes: list[LikeEventOut]
+    plays: list[PlayEventOut]
+    playlists: list[PlaylistSyncOut]
+    tracks: list[TrackOut]
+
+
+# -- push (client -> server) --------------------------------------------------
+class LikeEventIn(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    value: LikeValue
+    created_at: dt.datetime
+
+
+class PlayEventIn(BaseModel):
+    id: uuid.UUID
+    track_id: uuid.UUID
+    played_at: dt.datetime
+    play_duration_seconds: int | None = None
+    completed: bool = False
+
+
+class SyncPushIn(BaseModel):
+    likes: list[LikeEventIn] = Field(default_factory=list)
+    plays: list[PlayEventIn] = Field(default_factory=list)
+
+
+class SyncPushOut(BaseModel):
+    """How many events were newly stored (a replay reports 0) + a fresh cursor."""
+
+    cursor: dt.datetime
+    accepted_likes: int
+    accepted_plays: int