fix: use exact expected filename instead of newest-file heuristic

Parallel downloads to the same album dir (e.g. two tracks from same
album) both resolved to the last-written file, causing wrong metadata
to be embedded. Now each download targets its known output path
(trackPrefix + safeTitle + audioFmt) directly.

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

backend/novasonic-server.js

@@ -289,14 +289,10 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   // Without it yt-dlp gets a mixed feed of videos, channels, and playlists as well.
   const YTM_SONGS_PARAM = 'EgWKAQIIAWoKEAkQBRAKEAMQBA%3D%3D';
   const ytmUrl = `https://music.youtube.com/search?q=${encodeURIComponent(query)}&params=${YTM_SONGS_PARAM}`;
-  // Snapshot files already in dir before we start — so hasAudio() only detects NEW files
-  // from the current download attempt, not pre-existing files from parallel jobs.
-  let snapBefore = new Set(fs.readdirSync(dir));
-  const resetSnap = () => { snapBefore = new Set(fs.readdirSync(dir)); };
-  const hasAudio  = () => fs.readdirSync(dir).some(
-    f => !snapBefore.has(f) && AUDIO_EXTS.some(e => f.endsWith(`.${e}`))
-  );
-  const tryYtdlp  = async (args) => { resetSnap(); await spawnYtdlp(args); };
+  // We know the exact output filename before yt-dlp runs — no snapshot heuristic needed.
+  const expectedAudio = path.join(dir, `${trackPrefix}${safeTitle}.${CONFIG.audioFmt}`);
+  const hasAudio  = () => fs.existsSync(expectedAudio);
+  const tryYtdlp  = async (args) => { await spawnYtdlp(args); };
 
   const variantFilter  = buildVariantFilter(title);
   const titleKws       = buildTitleKeywords(title);
@@ -387,11 +383,8 @@ 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 audioFile = fs.readdirSync(dir)
-    .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];
+  // Step 2: locate the audio file yt-dlp wrote (exact expected path)
+  const audioFile = hasAudio() ? expectedAudio : null;
 
   if (!audioFile) {
     log(jobId, '⚠️  Keine Audio-Datei gefunden');