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/core/config.py

@@ -73,9 +73,15 @@ class Settings(BaseSettings):
     # -- external services (all optional; graceful degradation) ----------
     ml_service_url: str | None = None
     acoustid_api_key: SecretStr | None = None
+    acoustid_api_url: str = "https://api.acoustid.org/v2/lookup"
     musicbrainz_user_agent: str = "mcma-backend/0.1.0 ( https://github.com/your/repo )"
     youtube_cookies_path: Path | None = None
 
+    # -- enrichment -------------------------------------------------------
+    # ``fpcalc`` (Chromaprint) binary; resolved on PATH by default. The Docker
+    # image installs it via libchromaprint-tools.
+    fpcalc_path: str = "fpcalc"
+
     @field_validator("database_url")
     @classmethod
     def _require_async_driver(cls, v: str) -> str: