fix: rate-limit MB url-rels fetches, serialize frontend image loads

Artist image resolution was hitting musicbrainz.org with 10
concurrent unthrottled requests per search — enough to trigger
MB's rate limit rejection. Two fixes:

Backend: add dedicated 1 req/s RateLimiter for MB url-rels fetches
in ArtistImageProvider. Each fetch waits on the limiter before
the HTTP call. Results are cached 30 days so repeat lookups are
instant.

Frontend: switch loadArtistImages from concurrent fire-all to
sequential await loop. Each artist image loads one at a time,
images appear progressively as they resolve instead of all
failing from rate limit rejection.

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(cache, logger.WithGroup("artist-image"))
+	artistImg := NewArtistImageProvider(cache, NewRateLimiter(), logger.WithGroup("artist-image"))
 
 	logger.Info("explore service created")