fix(backend): treat yt-dlp exit code 101 as success

Code 101 means --max-downloads was reached — the file was downloaded
successfully. Treating it as error caused the ffmpeg/metadata step to
be skipped, leaving files with no tags.

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

backend/novasonic-server.js

@@ -244,7 +244,8 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     });
     proc.on('error', reject);
     proc.on('close', code =>
-      code === 0 ? resolve() : reject(new Error(`yt-dlp code ${code}: ${stderr.slice(-300)}`))
+      // 101 = --max-downloads reached (not an error — file was downloaded successfully)
+      (code === 0 || code === 101) ? resolve() : reject(new Error(`yt-dlp code ${code}: ${stderr.slice(-300)}`))
     );
   });