feat(tags): MusicBrainz artist-MBID tag fallback (#1519)

Widen keyless coverage: when a track's recording is untagged or has no
recording MBID, fall back to the artist's tags (/ws/2/artist/{mbid}
?inc=tags), down-weighted 0.6 as a coarser signal. Recording tags still
win outright when present.

- ListTracksMissingTags returns a.mbid AS artist_mbid; TrackRef gains
  ArtistMBID; the enricher threads it through.
- MB provider: recording-first, artist-fallback via a shared
  fetchEntityTags helper. A transient error at the recording step is
  returned (retry) rather than masked by the fallback.

Enricher, registry, and settings are untouched — the pluggable design
absorbs the wider lookup. Tests cover fallback-when-untagged,
artist-only-when-no-recording-MBID, and recording-preferred.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/db/dbq/track_tags.sql.go

@@ -112,7 +112,7 @@ func (q *Queries) ListPlayedTrackTagsForUser(ctx context.Context, arg ListPlayed
 
 const listTracksMissingTags = `-- name: ListTracksMissingTags :many
 
-SELECT t.id, t.mbid, t.title, a.name AS artist_name
+SELECT t.id, t.mbid, t.title, a.name AS artist_name, a.mbid AS artist_mbid
   FROM tracks t
   JOIN artists a ON a.id = t.artist_id
  WHERE t.tag_source IS NULL
@@ -131,6 +131,7 @@ type ListTracksMissingTagsRow struct {
 	Mbid       *string
 	Title      string
 	ArtistName string
+	ArtistMbid *string
 }
 
 // Track-tag enrichment queries (milestone #160, #1490). The tag enricher
@@ -140,7 +141,8 @@ type ListTracksMissingTagsRow struct {
 // Tracks eligible for tag enrichment: never processed (tag_source NULL)
 // or previously settled 'none' under an older provider version. Returns
 // the fields the provider chain needs — recording MBID (nullable) for
-// keyed lookups, plus title + artist name for name-based fallback.
+// keyed lookups, title + artist name for name-based fallback, and the
+// artist MBID (nullable) for MusicBrainz's artist-tag fallback (#1519).
 // $1 = current tag_sources_version, $2 = limit.
 func (q *Queries) ListTracksMissingTags(ctx context.Context, arg ListTracksMissingTagsParams) ([]ListTracksMissingTagsRow, error) {
 	rows, err := q.db.Query(ctx, listTracksMissingTags, arg.TagSourcesVersion, arg.Limit)
@@ -156,6 +158,7 @@ func (q *Queries) ListTracksMissingTags(ctx context.Context, arg ListTracksMissi
 			&i.Mbid,
 			&i.Title,
 			&i.ArtistName,
+			&i.ArtistMbid,
 		); err != nil {
 			return nil, err
 		}