fix: use ytmsearch: instead of ytmsearch3: (unsupported by yt-dlp)

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

backend/novasonic-server.js

@@ -230,7 +230,7 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   log(jobId, `🎵 YouTube Music: "${query}"`);
   const countBefore = audioFileCount();
   try {
-    await runYtdlp(`ytmsearch3:${query}`);
+    await runYtdlp(`ytmsearch:${query}`);
   } catch (e) {
     log(jobId, `⚠️  YouTube Music Fehler: ${e.message}`);
   }