fix: remove 'official audio' from yt-dlp search query

For remixes and edits, 'official audio' biases YouTube search towards the
artist's official channel (wrong song) instead of the specific version.
Duration filter is now the quality check — the plain query is more accurate.

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

backend/novasonic-server.js

@@ -179,7 +179,7 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   }
 
   const outputTemplate = path.join(dir, `${trackPrefix}${safeTitle}.%(ext)s`);
-  const query          = `${mainArtist} - ${title} official audio`;
+  const query          = `${mainArtist} - ${title}`;
 
   // Step 1: yt-dlp — audio only, no thumbnail (we embed iTunes artwork ourselves)
   const baseArgs = [