feat: cover-art pipeline (§1D)

Resolve, store and serve album cover art.

Sources (tag-first, mirroring enrichment): embedded artwork extracted
offline via mutagen (ID3 APIC / FLAC+OGG Picture / MP4 covr), then Cover
Art Archive by release-group MBID as a network fallback. Resolution runs
inside MetadataEnrichmentService after album resolution, only when the
album has no cover yet (idempotent, never overwrites), and is best-effort
so a cover failure never affects enrichment status.

- CoverArt value object + CoverArtExtractor/CoverArtProvider ports
- MutagenCoverExtractor + CoverArtArchiveClient adapters
- AcoustID parser now captures release_group_mbid
- Covers stored via FileStorage at covers/{album_id}.{ext} (local + S3)
- AlbumRepository.set_cover_path
- Serve real covers: GET /api/v1/albums|tracks/{id}/cover (StreamUser,
  ?token=), Subsonic getCoverArt (placeholder fallback)
- has_cover flag on AlbumOut/TrackOut
- coverart_enabled / coverart_base_url settings
- tests: cover resolution units + release_group parse + DB-backed
  test_cover_api.py (139 green via make test-api)

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

app/infrastructure/storage/s3.py

@@ -84,9 +84,7 @@ class S3FileStorage:
         async def _stream() -> AsyncGenerator[bytes]:
             async with self._client() as s3:
                 try:
-                    resp = await s3.get_object(
-                        Bucket=_bucket, Key=_key, Range=range_header
-                    )
+                    resp = await s3.get_object(Bucket=_bucket, Key=_key, Range=range_header)
                 except ClientError as exc:
                     raise StorageError(str(exc)) from exc
                 body = resp["Body"]