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/workers/arq_worker.py

@@ -1,7 +1,7 @@
 """arq worker settings — the queue runtime. Task functions register here.
 
 Run with: ``arq app.workers.arq_worker.WorkerSettings``.
-Tasks (download, enrich, transcode) are appended to ``functions`` in later steps.
+Tasks (download, transcode) are appended to ``functions`` in later steps.
 """
 
 from typing import Any, ClassVar
@@ -10,6 +10,7 @@ from arq.connections import RedisSettings
 
 from app.core.config import get_settings
 from app.core.logging import configure_logging, get_logger
+from app.workers.tasks.enrich_task import enrich_track
 from app.workers.tasks.import_task import scan_local_folder
 
 log = get_logger("worker")
@@ -26,7 +27,7 @@ async def shutdown(_ctx: dict[str, Any]) -> None:
 
 
 class WorkerSettings:
-    functions: ClassVar[list[Any]] = [scan_local_folder]
+    functions: ClassVar[list[Any]] = [scan_local_folder, enrich_track]
     on_startup = startup
     on_shutdown = shutdown
     max_jobs = get_settings().max_parallel_downloads