fix(backend): don't abort cascade on yt-dlp non-zero exit (e.g. video not available)

spawnYtdlp now always resolves so the hasAudio() cascade continues when a
single YTM result is geo-blocked or unavailable. Error is still logged.

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

backend/lib/ytdlp.js

@@ -173,10 +173,13 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
         .filter(l => l.trim() && !l.startsWith('[debug]'))
         .forEach(l => log(jobId, l.trim()));
     });
-    proc.on('error', reject);
-    proc.on('close', code =>
-      (code === 0 || code === 101) ? resolve() : reject(new Error(`yt-dlp code ${code}: ${stderr.slice(-300)}`))
-    );
+    proc.on('error', resolve);
+    proc.on('close', code => {
+      // Treat all non-zero exits as "no result" so the cascade continues.
+      // Fatal errors (network, bad args) are logged via stderr above; hasAudio() decides outcome.
+      if (code !== 0 && code !== 101) log(jobId, `⚠️  yt-dlp exit ${code}: ${stderr.slice(-200).trim()}`);
+      resolve();
+    });
   });
 
   const YTM_SONGS_PARAM = 'EgWKAQIIAWoKEAkQBRAKEAMQBA%3D%3D';