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/metadata/acoustid.py

@@ -66,7 +66,7 @@ class AcoustIdHttpClient:
                 )
             resp.raise_for_status()
             payload = resp.json()
-        except (httpx.HTTPError, ValueError):
+        except httpx.HTTPError, ValueError:
             log.warning("acoustid_lookup_failed")
             return None
 
@@ -100,6 +100,7 @@ def _parse_best_match(payload: object) -> RecordingMatch | None:
     score = float(best.get("score", 0.0))
 
     recording_mbid: str | None = None
+    release_group_mbid: str | None = None
     title: str | None = None
     artist: str | None = None
     album: str | None = None
@@ -115,13 +116,17 @@ def _parse_best_match(payload: object) -> RecordingMatch | None:
             artist = name if isinstance(name, str) else None
         groups = rec.get("releasegroups")
         if isinstance(groups, list) and groups and isinstance(groups[0], dict):
-            gtitle = groups[0].get("title")
+            group = groups[0]
+            gtitle = group.get("title")
             album = gtitle if isinstance(gtitle, str) else None
+            gid = group.get("id")
+            release_group_mbid = gid if isinstance(gid, str) else None
 
     return RecordingMatch(
         acoustid=acoustid,
         score=score,
         recording_mbid=recording_mbid,
+        release_group_mbid=release_group_mbid,
         title=title,
         artist=artist,
         album=album,