feat: download playlist tracks in parallel batches (MAX_PARALLEL)

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

backend/novasonic-server.js

@@ -831,18 +831,17 @@ app.post('/api/download', async (req, res) => {
         if (!plTracks.length) throw new Error('Keine Tracks übergeben');
         log(jobId, `🎵 Playlist "${title}" — ${plTracks.length} Tracks`);
         let done = 0, skipped = 0;
-        for (const track of plTracks) {
-          try {
+        const CONC = CONFIG.maxParallel || 2;
+        for (let i = 0; i < plTracks.length; i += CONC) {
+          const batch = plTracks.slice(i, i + CONC);
+          const batchResults = await Promise.allSettled(batch.map(async track => {
             const trackTitle   = track.trackName  || '';
             // subtitle gives "Artist1, Artist2" — use first for iTunes search, keep full for tags
             const trackArtist  = track.artistName || artist || '';
             const searchArtist = trackArtist.split(',')[0].trim();
-
-            // Always look up iTunes for correct album, artwork, year (Spotify flat format has none)
             log(jobId, `📋 iTunes: "${trackArtist} – ${trackTitle}"`);
             const itunes = await lookupItunesTrackMeta(searchArtist, trackTitle);
-
-            const result = await downloadTrack({
+            return downloadTrack({
               jobId,
               artist:      trackArtist,
               albumArtist: trackArtist,
@@ -856,10 +855,17 @@ app.post('/api/download', async (req, res) => {
               explicit:    track.trackExplicitness === 'explicit',
               durationMs:  itunes.durationMs  || track.durationMs    || undefined,
             });
-            if (result?.skipped) skipped++; else done++;
+          }));
+          for (let j = 0; j < batchResults.length; j++) {
+            const r = batchResults[j];
+            if (r.status === 'rejected') {
+              log(jobId, `⚠️  Fehler bei "${batch[j].trackName || '?'}": ${r.reason?.message || r.reason}`);
+            } else if (r.value?.skipped) {
+              skipped++;
+            } else {
+              done++;
+            }
             updateJob(jobId, { progress: Math.round(((done + skipped) / plTracks.length) * 90) });
-          } catch(e) {
-            log(jobId, `⚠️  Fehler bei "${track.trackName || '?'}": ${e.message}`);
           }
         }
         const allSkipped = skipped === plTracks.length;