fix: also write totalTracks, discNum, discTotal for Spotify imports

Spotify enrichment only copies album/trackNum/duration from iTunes.
Now also fetch totalTracks, discNum, discTotal from iTunes for:
- alreadyEnriched playlist tracks
- single track downloads where album came from Spotify

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

backend/novasonic-server.js

@@ -424,10 +424,13 @@ app.post('/api/download', async (req, res) => {
           meta = { ...meta, ...extra, artworkUrl: spArt || extra.artworkUrl || '', genre: meta.genre || extra.genre || undefined };
         } else {
           log(jobId, `📋 Album aus Spotify: "${meta.album}"`);
-          // Spotify has no per-track genre — still look up genre from iTunes
-          if (!meta.genre) {
+          // Spotify enrichment has no genre, totalTracks, or disc info — fetch from iTunes
+          if (!meta.genre || !meta.totalTracks) {
             const genreMeta = await lookupItunesTrackMeta(searchArtist, title).catch(() => ({}));
-            if (genreMeta.genre) { meta.genre = genreMeta.genre; log(jobId, `📋 Genre: "${genreMeta.genre}"`); }
+            if (!meta.genre       && genreMeta.genre)       { meta.genre       = genreMeta.genre;       log(jobId, `📋 Genre: "${genreMeta.genre}"`); }
+            if (!meta.totalTracks && genreMeta.totalTracks) { meta.totalTracks = genreMeta.totalTracks; }
+            if (!meta.discNum     && genreMeta.discNum)     { meta.discNum     = genreMeta.discNum;     }
+            if (!meta.discTotal   && genreMeta.discTotal)   { meta.discTotal   = genreMeta.discTotal;   }
           }
         }
         const finalArtist = meta.itunesArtistName || artist;
@@ -501,9 +504,14 @@ app.post('/api/download', async (req, res) => {
             let itunes = {};
             if (alreadyEnriched) {
               log(jobId, `📋 Frontend-Metadaten: "${trackArtist} – ${trackTitle}" (${track.collectionName})`);
-              // Still look up genre — frontend enrichment doesn't include it and Spotify has no genre per track
+              // Spotify enrichment copies album/trackNum/duration but not genre/totalTracks/disc — fetch those
               const genreMeta = await lookupItunesTrackMeta(searchArtist, trackTitle).catch(() => ({}));
-              itunes = { genre: genreMeta.genre };
+              itunes = {
+                genre:       genreMeta.genre,
+                totalTracks: genreMeta.totalTracks,
+                discNum:     genreMeta.discNum,
+                discTotal:   genreMeta.discTotal,
+              };
             } else {
               log(jobId, `📋 iTunes: "${trackArtist} – ${trackTitle}"`);
               itunes = await lookupItunesTrackMeta(searchArtist, trackTitle);