feat: 'In Library' badges, artist images on local pages, MBID-based Tier 3

Three features wired together:

1. 'In Library' badges on explore search results:
   CheckLibraryMBIDs Wails binding batch-checks which search result
   MBIDs exist in the local library. Green badges render on matching
   artist cards and album cards.

2. Artist images on local artist-details page:
   Local artist pages now call GetArtistMBID(name) to resolve the
   MBID from tags, then GetArtistImageURL(mbid) to fetch the cached
   Wikimedia photo. Falls back to initial-letter avatar.

3. Tier 3 search index uses direct MBIDs from tags:
   buildTier3Library now reads artists.mbid column (from audio tags)
   for direct MBID matching, falling back to name matching for
   untagged artists. Eliminates false matches and catches artists
   that name matching misses.

backend/database/sql/sqlcgen/models.go

@@ -49,6 +49,15 @@ type CoverArt struct {
 	MimeType   string
 }
 
+type ExploreCache struct {
+	UrlKey     string
+	Response   string
+	Mbid       sql.NullString
+	EntityType sql.NullString
+	ExpiresAt  time.Time
+	CreatedAt  time.Time
+}
+
 type FileType struct {
 	ID        int64
 	Extension string