fix: simplify artist image provider, remove unnecessary LookupArtist call

The previous implementation called LookupArtist (rate-limited MB API)
before fetching url-rels, wasting a rate limiter slot. The fetchURL
for rels also bypassed the MB rate limiter, risking 503 rejections.

Rewrite: fetch MB url-rels once (direct HTTP, cached 30 days), parse
both image and wikidata relations from the same response, resolve
Wikimedia thumb URL. No dependency on MusicBrainzClient — just the
Cache for storage and a plain http.Client.

Also cleared 34 stale cached empty results from previous failed
resolution attempts that were blocking image lookup.

backend/explore/explore.go

@@ -38,7 +38,7 @@ func NewExploreService(logger *slog.Logger, db *database.DB) *Service {
 	lb := NewListenBrainzClient(limiter, cache, logger.WithGroup("listenbrainz"))
 	index := NewSearchIndex(db, lb, logger.WithGroup("search-index"))
 	artProxy := NewCoverArtProxy(db, limiter)
-	artistImg := NewArtistImageProvider(mb, cache, logger.WithGroup("artist-image"))
+	artistImg := NewArtistImageProvider(cache, logger.WithGroup("artist-image"))
 
 	logger.Info("explore service created")