fix(backend): direct-link download failure was reported as success

The direct-URL (sourceUrl) path returned { success: true } even when no audio
file was produced (e.g. YouTube 403 / bot check), so the job was marked
"done/Fertig!" and web + Android both said the track was added. It now returns
{ failed: true } → the server sets status 'error' with a clear message, which
both frontends already render as a failure. Search-cascade path was already
correct.

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

backend/lib/ytdlp.js

@@ -198,7 +198,10 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
     log(jobId, `🔗 Direkter Download: ${sourceUrl}`);
     await spawnYtdlp([sourceUrl, '--no-playlist', ...baseArgs]);
     const audioFile = hasAudio() ? expectedAudio : null;
-    if (!audioFile) { log(jobId, '⚠️  Download fehlgeschlagen'); return { success: true, dir }; }
+    // Direct-link download failed (e.g. YouTube 403 / bot check) — must report FAILURE, not success.
+    // Previously returned { success: true }, so the job was marked "done/Fertig!" and the UI wrongly
+    // said the track was added.
+    if (!audioFile) { log(jobId, '⚠️  Download fehlgeschlagen'); return { success: false, failed: true, dir }; }
     return embedMetadata({ jobId, audioFile, dir, trackPrefix, safeTitle, rawArtist, artist, mainArtist, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, title: searchTitle });
   }