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/queries/track_tags.sql

@@ -7,9 +7,10 @@
 -- 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.
-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