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/v1/sync.py

@@ -1,15 +1,103 @@
-"""Client sync endpoints (offline-first event log)."""
+"""Client sync endpoints (offline-first event log).
 
-from typing import Any
+``GET /sync/changes`` pulls everything the caller changed since their cursor;
+``POST /sync/push`` uploads the like/play events a client accumulated offline
+(idempotent — replays are no-ops). See :mod:`app.application.sync_service`.
+"""
+
+import datetime as dt
 
 from fastapi import APIRouter
 
+from app.api.deps import AlbumRepoDep, ArtistRepoDep, CurrentUser, SyncServiceDep
+from app.api.schemas.sync import (
+    LikeEventOut,
+    PlayEventOut,
+    PlaylistSyncOut,
+    SyncChangesOut,
+    SyncPushIn,
+    SyncPushOut,
+)
+from app.api.v1.tracks import _build_track_out
+from app.application.sync_service import LikeEvent, PlayEvent
+
 router = APIRouter(prefix="/sync", tags=["sync"])
 
 
 @router.get("/changes")
-async def get_changes() -> Any: ...
+async def get_changes(
+    service: SyncServiceDep,
+    artist_repo: ArtistRepoDep,
+    album_repo: AlbumRepoDep,
+    user: CurrentUser,
+    since: dt.datetime | None = None,
+) -> SyncChangesOut:
+    """Delta since ``since`` (omit for a full snapshot). Persist ``cursor`` from
+    the response and pass it back as ``?since=`` next time."""
+    changes = await service.get_changes(user.id, since=since)
+
+    artist_ids = list({t.artist_id for t in changes.tracks})
+    album_ids = list({t.album_id for t in changes.tracks if t.album_id is not None})
+    artists = {a.id: a for a in await artist_repo.get_many(artist_ids)}
+    albums = {a.id: a for a in await album_repo.get_many(album_ids)}
+    tracks_out = await _build_track_out(changes.tracks, artists, albums)
+
+    return SyncChangesOut(
+        cursor=changes.cursor,
+        likes=[
+            LikeEventOut(
+                id=lk.id, track_id=lk.track_id, value=lk.value, created_at=lk.created_at
+            )
+            for lk in changes.likes
+        ],
+        plays=[
+            PlayEventOut(
+                id=p.id,
+                track_id=p.track_id,
+                played_at=p.played_at,
+                play_duration_seconds=p.play_duration_seconds,
+                completed=p.completed,
+            )
+            for p in changes.plays
+        ],
+        playlists=[
+            PlaylistSyncOut(
+                id=d.playlist.id,
+                name=d.playlist.name,
+                description=d.playlist.description,
+                version=d.playlist.version,
+                updated_at=d.playlist.updated_at,
+                track_ids=d.track_ids,
+            )
+            for d in changes.playlists
+        ],
+        tracks=tracks_out,
+    )
 
 
 @router.post("/push")
-async def push_changes() -> Any: ...
+async def push_changes(
+    body: SyncPushIn, service: SyncServiceDep, user: CurrentUser
+) -> SyncPushOut:
+    result = await service.push(
+        user.id,
+        likes=[
+            LikeEvent(id=e.id, track_id=e.track_id, value=e.value, created_at=e.created_at)
+            for e in body.likes
+        ],
+        plays=[
+            PlayEvent(
+                id=e.id,
+                track_id=e.track_id,
+                played_at=e.played_at,
+                play_duration_seconds=e.play_duration_seconds,
+                completed=e.completed,
+            )
+            for e in body.plays
+        ],
+    )
+    return SyncPushOut(
+        cursor=result.cursor,
+        accepted_likes=result.accepted_likes,
+        accepted_plays=result.accepted_plays,
+    )