fix(backend): tighten duration filter margin from ±15% to ±10% (min 10s)

Matches spotdl behaviour; still covers YouTube intros/outros while
reducing false positives from similarly-lengthed wrong songs.

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

backend/lib/ytdlp.js

@@ -253,7 +253,7 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
 
   if (durationMs && durationMs > 30000) {
     const durationSec = Math.round(durationMs / 1000);
-    const margin = Math.max(15, Math.round(durationSec * 0.15));
+    const margin = Math.max(10, Math.round(durationSec * 0.10));
     const dur = { min: Math.max(30, durationSec - margin), max: durationSec + margin };
 
     log(jobId, `🔍 Suche YT Music (~${durationSec}s ±${margin}s): "${query}"`);