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.

frontend/src/components/explore-view/explore-view.ts

@@ -666,25 +666,25 @@ export class ExploreView extends LitElement {
      * Load artist images for all visible artist cards.  Each call
      * is async and updates the cache + re-renders on success.
      */
-    private loadArtistImages() {
+    private async loadArtistImages() {
         if (!this.results?.artists?.length) return;
 
+        // Load sequentially to avoid hammering the MB rate limiter.
         for (const a of this.results.artists) {
             if (this.artistImageCache.has(a.mbid)) continue;
 
-            // Mark as loading.
             this.artistImageCache.set(a.mbid, '');
 
-            GetArtistImageURL(a.mbid)
-                .then((url) => {
-                    if (url) {
-                        this.artistImageCache.set(a.mbid, url);
-                        this.requestUpdate();
-                    }
-                })
-                .catch(() => {
-                    // No image — leave empty string.
-                });
+            try {
+                const url = await GetArtistImageURL(a.mbid);
+
+                if (url) {
+                    this.artistImageCache.set(a.mbid, url);
+                    this.requestUpdate();
+                }
+            } catch {
+                // No image — leave empty string.
+            }
         }
     }