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/artistimage.go

@@ -40,21 +40,24 @@ const (
 // URL.  Designed to be extended with additional sources (fanart.tv,
 // etc.) by adding to the resolve chain.
 type ArtistImageProvider struct {
-	cache  *Cache
-	client *http.Client
-	logger *slog.Logger
+	cache     *Cache
+	mbLimiter *RateLimiter
+	client    *http.Client
+	logger    *slog.Logger
 }
 
 // NewArtistImageProvider creates a provider that resolves artist
 // images via MusicBrainz relationships and Wikidata.
 func NewArtistImageProvider(
 	cache *Cache,
+	mbLimiter *RateLimiter,
 	logger *slog.Logger,
 ) *ArtistImageProvider {
 	return &ArtistImageProvider{
-		cache:  cache,
-		client: &http.Client{Timeout: artistImageTimeout},
-		logger: logger,
+		cache:     cache,
+		mbLimiter: mbLimiter,
+		client:    &http.Client{Timeout: artistImageTimeout},
+		logger:    logger,
 	}
 }
 
@@ -130,6 +133,11 @@ func (p *ArtistImageProvider) fetchMBRels(artistMBID string) []mbRelation {
 		artistMBID,
 	)
 
+	// Rate-limit the MB API call.
+	if err := p.mbLimiter.Wait(context.Background()); err != nil {
+		return nil
+	}
+
 	body, err := p.fetchURL(url)
 	if err != nil {
 		p.logger.Debug("artist image: MB rels fetch failed",

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")