fix: handle iTunes rate-limit empty responses, reduce Spotify artwork concurrency

- itunesSearch: use r.text() + JSON.parse so empty body (rate-limit) returns [] instead of throwing
- reduce Spotify artwork batch size 8→4 to avoid hammering iTunes and affecting global search

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/spotify.js

@@ -172,7 +172,7 @@ async function _loadSpotifyTrackArtwork(tracks, listEl) {
   });
 
   // Enrich each track with iTunes data — same call as global search
-  const CONC = 8;
+  const CONC = 4;
   for (let i = 0; i < tracks.length; i += CONC) {
     const batch = tracks.slice(i, i + CONC);
     await Promise.all(batch.map(async (t, bi) => {