Add duplicate check before downloading tracks

Before starting yt-dlp, check if Artist/Album/Title.{ext} already
exists on disk. If found, log a skip message and return early instead
of re-downloading and overwriting the existing file.

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

backend/novasonic-server.js

@@ -107,6 +107,14 @@ async function downloadArtwork(url, dir) {
   });
 }
 
+const AUDIO_EXTS = ['mp3', 'flac', 'opus', 'm4a', 'ogg', 'wav'];
+
+function findExistingTrack(dir, trackPrefix, safeTitle) {
+  return AUDIO_EXTS
+    .map(ext => path.join(dir, `${trackPrefix}${safeTitle}.${ext}`))
+    .find(f => fs.existsSync(f)) || null;
+}
+
 // Download a single track: yt-dlp audio + ffmpeg clean metadata + iTunes cover embedded
 async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, outputDir }) {
   const mainArtist = albumArtist || artist;
@@ -117,6 +125,13 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
 
   const safeTitle   = sanitize(title);
   const trackPrefix = trackNum ? String(trackNum).padStart(2, '0') + ' - ' : '';
+
+  const existing = findExistingTrack(dir, trackPrefix, safeTitle);
+  if (existing) {
+    log(jobId, `⏭ Bereits vorhanden: ${path.basename(existing)} — übersprungen`);
+    return { success: true, dir, skipped: true };
+  }
+
   const outputTemplate = path.join(dir, `${trackPrefix}${safeTitle}.%(ext)s`);
   const query          = `${mainArtist} - ${title} official audio`;
   log(jobId, `🔍 Suche: "${query}"`);
@@ -156,9 +171,8 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   });
 
   // Step 2: find the audio file yt-dlp wrote (newest audio file in dir)
-  const audioExts = ['mp3', 'flac', 'opus', 'm4a', 'ogg', 'wav'];
   const audioFile = fs.readdirSync(dir)
-    .filter(f => audioExts.some(e => f.endsWith(`.${e}`)))
+    .filter(f => AUDIO_EXTS.some(e => f.endsWith(`.${e}`)))
     .map(f => path.join(dir, f))
     .sort((a, b) => fs.statSync(b).mtimeMs - fs.statSync(a).mtimeMs)[0];