fix(backend): add ytmsearch5 step + expand YTM pool to 20 for better coverage

ytmsearch5 (yt-dlp native YTM search) added as step 2 — returns different
result ordering than the manual YTM URL, recovering songs that the URL
approach ranks too low. YTM pool expanded from 10 → 20 results to reduce
fallthrough to YouTube.

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

backend/lib/ytdlp.js

@@ -247,7 +247,7 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
   // Fetch YTM URLs once upfront so every cascade step filters the SAME result set.
   // Re-fetching produced different orderings, causing the correct video (found in step 1
   // but failing the channel filter) to disappear by step 3 and a wrong one to slip through.
-  const ytmUrls = await getYtmVideoUrls(10);
+  const ytmUrls = await getYtmVideoUrls(20);
   const tryYtm = async (n, mfArgs) => {
     const urls = n ? ytmUrls.slice(0, n) : ytmUrls;
     if (!urls.length) return;
@@ -260,47 +260,51 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
     const dur = { min: Math.max(30, durationSec - margin), max: durationSec + margin };
 
     log(jobId, `🔍 Suche YT Music (~${durationSec}s ±${margin}s): "${query}"`);
-    // 1. YTM top-5 — strict: duration + title + variant + channel check
+    // 1. YTM top-5 via manual URL — strict: duration + title + variant + channel check
     await tryYtm(5, mf(dur, true, true));
     if (!hasAudio()) {
-      // 2. YT top-5 — strict: same filters including channel check
+      // 2. ytmsearch5 (yt-dlp native YTM) — different result ordering than manual URL
+      await spawnYtdlp([`ytmsearch5:${query}`, ...PI_FILTERED, ...mf(dur, true, true), ...baseArgs]);
+    }
+    if (!hasAudio()) {
+      // 3. YT top-5 — strict: same filters including channel check
       log(jobId, '⚠️  Kein YTM-Treffer — Fallback zu YouTube…');
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(dur, true, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 3. YTM all-10 — no channel check (label channels, etc.)
+      // 4. YTM all-20 — no channel check (label channels, etc.)
-      log(jobId, '⚠️  Erweitere Suche auf top 10 mit Dauerbeschränkung…');
+      log(jobId, '⚠️  Erweitere Suche auf top 20 mit Dauerbeschränkung…');
       await tryYtm(0, mf(dur, true));
     }
     if (!hasAudio()) {
-      // 4. YT top-10 — no channel check
+      // 5. YT top-10 — no channel check
       await spawnYtdlp([`ytsearch10:${query}`, ...PI_FILTERED_10, ...mf(dur, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 5. YTM all — no duration check
+      // 6. YTM all — no duration check
       log(jobId, '⚠️  Kein Treffer im Dauerbereich — suche ohne Dauerbeschränkung…');
       await tryYtm(0, mf(null, true));
     }
     if (!hasAudio()) {
-      // 6. YT top-5 — no duration check
+      // 7. YT top-5 — no duration check
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 7. YTM all — title only (no variant filter)
+      // 8. YTM all — title only (no variant filter)
       log(jobId, '⚠️  Letzter gefilterter Versuch…');
       await tryYtm(0, mf(null, false));
     }
     if (!hasAudio()) {
-      // 8. YT top-5 — title only
+      // 9. YT top-5 — title only
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, false), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 9. YTM — no filter at all, take first video result
+      // 10. YTM — no filter at all, take first video result
       log(jobId, '⚠️  Letzter Versuch: keine Filter…');
       await tryYtm(3, []);
     }
     if (!hasAudio()) {
-      // 10. YT — no filter
+      // 11. YT — no filter
       await spawnYtdlp([`ytsearch1:${query}`, ...PI_UNFILTERED, ...baseArgs]);
     }
   } else {