feat(enrichment): tag-first metadata pipeline (§1D)

Implements the §6.2 enrichment pipeline: embedded tags → Chromaprint
fingerprint → AcoustID lookup. Well-tagged files get correct
artist/album/title offline; the rest are identified via AcoustID
(which also yields a MusicBrainz recording id in one call).

- domain: AudioTags/Fingerprint/RecordingMatch value objects; ports
  AudioTagReader, AudioFingerprinter, AcoustIdClient; TrackRepository
  .apply_enrichment (gap-fill, never erases) + AlbumRepository.get_or_create
- infrastructure/metadata: MutagenTagReader, FpcalcFingerprinter,
  AcoustIdHttpClient (rich meta=recordings+releasegroups, throttled)
- application: MetadataEnrichmentService — tags preferred, AcoustID fills
  gaps; resolves artist/album; status enriched/failed; skips manual;
  every external step wrapped (graceful degradation)
- workers: enrich_task registered; enqueue_enrich is best-effort and
  deferred so the caller's txn commits before the worker reads the row
- wiring: upload enqueues after add; import returns imported_ids and
  enqueues post-commit (mid-scan would race the worker); manual
  POST /tracks/{id}/metadata/enrich endpoint
- deps: add mutagen (fpcalc/ffmpeg already in the image)

Tests: metadata service orchestration, AcoustID parser, tag helpers.
125 passed; mypy strict + ruff clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/v1/tracks.py

@@ -11,6 +11,7 @@ from app.api.schemas.track import TrackOut, TrackUpdate
 from app.domain.entities.album import Album
 from app.domain.entities.track import Artist, Track
 from app.domain.errors import NotFoundError
+from app.workers.queue import enqueue
 
 router = APIRouter(prefix="/tracks", tags=["tracks"])
 
@@ -147,7 +148,18 @@ async def get_track_cover(track_id: uuid.UUID, _: CurrentUser) -> Any: ...
 
 
 @router.post("/{track_id}/metadata/enrich")
-async def enrich_metadata(track_id: uuid.UUID, _: CurrentUser) -> Any: ...
+async def enrich_metadata(
+    track_id: uuid.UUID,
+    track_repo: TrackRepoDep,
+    _: CurrentUser,
+) -> dict[str, str]:
+    """Re-run metadata enrichment for a track (admin/user-triggered). The work
+    happens in a worker; this only enqueues it. 503 if the queue is down."""
+    track = await track_repo.get_by_id(track_id)
+    if track is None:
+        raise NotFoundError(f"Track {track_id} not found.")
+    job_id = await enqueue("enrich_track", track_id=str(track_id))
+    return {"track_id": str(track_id), "job_id": job_id}
 
 
 @router.get("/{track_id}/metadata/matches")