fix(backend): use web_music player client for YTM track access

yt-dlp's android_vr player client returns "This video is not available" for
many YouTube Music Art Track / topic uploads (e.g. MINA - recognize) that
play fine in the browser. Switch to player_client=web_music with default
fallback so these tracks become downloadable, fixing the cascade falling
through to wrong YouTube videos.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/lib/ytdlp.js

@@ -155,6 +155,10 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
     '--no-playlist',
     '--no-warnings',
     '--newline',
+    // Use the web_music player client (falling back to default) so YouTube Music
+    // "Art Track" / topic uploads are reachable. The android_vr client returns
+    // "This video is not available" for many YTM tracks that play fine in the browser.
+    '--extractor-args', 'youtube:player_client=web_music,default',
   ];
 
   const spawnYtdlp = (args) => new Promise((resolve, reject) => {