fix: fetch YTM URLs once and reuse across cascade steps

Each tryYtmSearch() call made a fresh HTTP request, so YouTube could
return different result orderings between steps. The correct video
(right title, wrong channel name) appeared in step 1 but was gone by
step 3 — a worse video with matching keywords slipped through instead.

Now fetch 10 YTM URLs once upfront and filter the same list with
progressively relaxed rules. Also saves 2-4 extra YTM requests per
download.

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

backend/lib/ytdlp.js

@@ -145,15 +145,6 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     proc.on('close', () => resolve(channelExpanded ? [] : urls));
   });
 
-  // Search YTM for video URLs (pre-filtered), then pass them all at once to yt-dlp.
-  // --max-downloads 1 makes yt-dlp stop after the first video that passes mfArgs.
-  // Filtered/skipped videos do NOT count toward --max-downloads, so the cascade works.
-  const tryYtmSearch = async (maxItems, mfArgs) => {
-    const urls = await getYtmVideoUrls(maxItems);
-    if (!urls.length) return;
-    await spawnYtdlp([...urls, '--max-downloads', '1', ...mfArgs, ...baseArgs]);
-  };
-
   const variantFilter = buildVariantFilter(title);
   const titleKws      = buildTitleKeywords(title);
   const variantPart   = variantFilter ? ` & ${variantFilter}` : '';
@@ -177,6 +168,16 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   const PI_FILTERED_10 = ['--playlist-items', '1-10', '--max-downloads', '1'];
   const PI_UNFILTERED  = ['--playlist-items', '1'];
 
+  // 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 tryYtm = async (n, mfArgs) => {
+    const urls = n ? ytmUrls.slice(0, n) : ytmUrls;
+    if (!urls.length) return;
+    await spawnYtdlp([...urls, '--max-downloads', '1', ...mfArgs, ...baseArgs]);
+  };
+
   if (durationMs) {
     const durationSec = Math.round(durationMs / 1000);
     const margin = Math.max(15, Math.round(durationSec * 0.15));
@@ -184,34 +185,34 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
 
     log(jobId, `🔍 Suche YT Music (~${durationSec}s ±${margin}s): "${query}"`);
     // 1. YTM top-5 — strict: duration + title + variant + channel check
-    await tryYtmSearch(5, mf(dur, true, true));
+    await tryYtm(5, mf(dur, true, true));
     if (!hasAudio()) {
       // 2. YT top-5 — strict: same filters including channel check
       log(jobId, '⚠️  Kein YTM-Treffer — Fallback zu YouTube…');
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(dur, true, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 3. YTM top-10 — no channel check (label channels, etc.)
+      // 3. YTM all-10 — no channel check (label channels, etc.)
       log(jobId, '⚠️  Erweitere Suche auf top 10 mit Dauerbeschränkung…');
-      await tryYtmSearch(10, mf(dur, true));
+      await tryYtm(0, mf(dur, true));
     }
     if (!hasAudio()) {
       // 4. YT top-10 — no channel check
       await spawnYtdlp([`ytsearch10:${query}`, ...PI_FILTERED_10, ...mf(dur, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 5. YTM top-5 — no duration check
+      // 5. YTM all — no duration check
       log(jobId, '⚠️  Kein Treffer im Dauerbereich — suche ohne Dauerbeschränkung…');
-      await tryYtmSearch(5, mf(null, true));
+      await tryYtm(0, mf(null, true));
     }
     if (!hasAudio()) {
       // 6. YT top-5 — no duration check
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 7. YTM top-5 — title only (no variant filter)
+      // 7. YTM all — title only (no variant filter)
       log(jobId, '⚠️  Letzter gefilterter Versuch…');
-      await tryYtmSearch(5, mf(null, false));
+      await tryYtm(0, mf(null, false));
     }
     if (!hasAudio()) {
       // 8. YT top-5 — title only
@@ -220,7 +221,7 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     if (!hasAudio()) {
       // 9. YTM — no filter at all, take first video result
       log(jobId, '⚠️  Letzter Versuch: keine Filter…');
-      await tryYtmSearch(3, []);
+      await tryYtm(3, []);
     }
     if (!hasAudio()) {
       // 10. YT — no filter
@@ -229,23 +230,23 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   } else {
     log(jobId, `🔍 Suche YT Music: "${query}"`);
     // 1. YTM top-5 — strict: title + variant + channel check
-    await tryYtmSearch(5, mf(null, true, true));
+    await tryYtm(5, mf(null, true, true));
     if (!hasAudio()) {
       // 2. YT top-5 — strict
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, true, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 3. YTM top-5 — no channel check
+      // 3. YTM all-10 — no channel check (catches label/distributor channels)
-      await tryYtmSearch(5, mf(null, true));
+      await tryYtm(0, mf(null, true));
     }
     if (!hasAudio()) {
       // 4. YT top-5 — no channel check
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 5. YTM top-5 — title only
+      // 5. YTM all — title only
       log(jobId, '⚠️  Letzter gefilterter Versuch…');
-      await tryYtmSearch(5, mf(null, false));
+      await tryYtm(0, mf(null, false));
     }
     if (!hasAudio()) {
       // 6. YT top-5 — title only
@@ -254,7 +255,7 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     if (!hasAudio()) {
       // 7. YTM — first video result, no filter
       log(jobId, '⚠️  Letzter Versuch: keine Filter…');
-      await tryYtmSearch(3, []);
+      await tryYtm(3, []);
     }
     if (!hasAudio()) {
       // 8. YT — no filter