feat(spotify): lazy-load per-track iTunes artwork for playlists/albums

Adds POST /api/spotify/track-artwork endpoint that takes an array of
{idx, trackName, artistName} and returns iTunes artwork URLs in batches
of 8 concurrent requests.

Frontend calls this after rendering the track list, then patches each
.gtrack-art element with the correct album cover as results arrive.
Also updates _spotifyResolved.tracks so downloads use the right artwork.

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

backend/novasonic-server.js

@@ -484,6 +484,22 @@ function _spTrackItem(t, defaultAlbum, defaultArt, defaultAlbumArtist) {
 
 // ─── ROUTES ──────────────────────────────────────────────────────────────────
 
+app.post('/api/spotify/track-artwork', async (req, res) => {
+  const tracks = req.body.tracks || [];
+  if (!tracks.length) return res.json({ results: [] });
+  const CONC = 8;
+  const results = [];
+  for (let i = 0; i < tracks.length; i += CONC) {
+    const batch = tracks.slice(i, i + CONC);
+    const batchRes = await Promise.all(batch.map(async t => {
+      const meta = await lookupItunesTrackMeta(t.artistName || '', t.trackName || '');
+      return { idx: t.idx, artworkUrl: meta.artworkUrl || '' };
+    }));
+    results.push(...batchRes);
+  }
+  res.json({ results });
+});
+
 app.get('/api/health', (req, res) => {
   res.json({ status: 'ok', ytdlpVersion, musicPath: CONFIG.musicPath, audioFormat: CONFIG.audioFmt, activeJobs: activeDownloads, queueLength: downloadQueue.length });
 });