fix(backend): fix getYtmVideoUrls always returning empty + remove unsupported ytmsearch5

[youtube:tab] in stderr fires for all YTM search page processing, not just
artist channel expansion, causing channelExpanded=true for every search and
returning [] — bypassing all YTM cascade steps entirely. Fix: only check
stdout for /browse/UC (actual channel URL in results). Remove ytmsearch5:
scheme which is not supported by this yt-dlp version. Revert pool to 10.

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

backend/lib/ytdlp.js

@@ -202,16 +202,14 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
   // actual video URLs (watch?v=…). This prevents yt-dlp from following an artist channel
   // result and treating its uploads as the search result set.
   //
-  // yt-dlp expands "url_transparent" artist channel entries even under --flat-playlist,
-  // printing the channel's video URLs instead of the channel URL. We detect this by
-  // watching for [youtube:tab] in output (which only fires during channel/tab extraction)
-  // and return [] so the caller falls through to the ytsearch fallback.
+  // When YTM returns an artist channel as the top hit, yt-dlp prints the channel's
+  // browse URL (/browse/UC…) instead of video URLs. We detect this on stdout and
+  // return [] so the caller falls through to the ytsearch fallback.
+  // NOTE: do NOT check for [youtube:tab] in stderr — that fires for all YTM search
+  // page processing, not just channel expansion, and would wrongly return [] always.
   const getYtmVideoUrls = (maxItems) => new Promise(resolve => {
     const urls = [];
     let channelExpanded = false;
-    const checkExpansion = (text) => {
-      if (text.includes('[youtube:tab]') || text.includes('/browse/UC')) channelExpanded = true;
-    };
     const proc = spawn(CONFIG.ytdlpBin, [
       ytmUrl,
       '--flat-playlist',
@@ -221,7 +219,7 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
     ], { env: { ...process.env, PYTHONUNBUFFERED: '1' } });
     proc.stdout.on('data', d => {
       const text = d.toString();
-      checkExpansion(text);
+      if (text.includes('/browse/UC')) { channelExpanded = true; return; }
       if (!channelExpanded) {
         text.split('\n').forEach(line => {
           const u = line.trim();
@@ -229,9 +227,12 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
         });
       }
     });
-    proc.stderr.on('data', d => checkExpansion(d.toString()));
     proc.on('error', () => resolve([]));
-    proc.on('close', () => resolve(channelExpanded ? [] : urls));
+    proc.on('close', () => {
+      if (channelExpanded) log(jobId, `⚠️  YTM: Artist-Channel in Suchergebnissen — überspringe`);
+      else log(jobId, `📡 YTM: ${urls.length} Ergebnis(se) gefunden`);
+      resolve(channelExpanded ? [] : urls);
+    });
   });
 
   const mf = (dur, variant, strict = false) => {
@@ -247,7 +248,7 @@ 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(20);
+  const ytmUrls = await getYtmVideoUrls(10);
   const tryYtm = async (n, mfArgs) => {
     const urls = n ? ytmUrls.slice(0, n) : ytmUrls;
     if (!urls.length) return;
@@ -260,24 +261,20 @@ 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): "${query}"`);
-    // 1. YTM top-5 via manual URL — strict: duration + title + variant + channel check
+    // 1. YTM top-5 — strict: duration + title + variant + channel check
     await tryYtm(5, mf(dur, true, true));
     if (!hasAudio()) {
-      // 2. ytmsearch5 (yt-dlp native YTM) — different result ordering than manual URL
-      await spawnYtdlp([`ytmsearch5:${query}`, ...PI_FILTERED, ...mf(dur, true, true), ...baseArgs]);
-    }
-    if (!hasAudio()) {
-      // 3. YT top-5 — strict: same filters including channel check
+      // 2. YT top-5 — strict: same filters including channel check
       log(jobId, '⚠️  Kein YTM-Treffer — Fallback zu YouTube…');
       await spawnYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(dur, true, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      // 4. YTM all-20 — no channel check (label channels, etc.)
-      log(jobId, '⚠️  Erweitere Suche auf top 20 mit Dauerbeschränkung…');
+      // 3. YTM all-10 — no channel check (label channels, etc.)
+      log(jobId, '⚠️  Erweitere Suche auf top 10 mit Dauerbeschränkung…');
       await tryYtm(0, mf(dur, true));
     }
     if (!hasAudio()) {
-      // 5. YT top-10 — no channel check
+      // 4. YT top-10 — no channel check
       await spawnYtdlp([`ytsearch10:${query}`, ...PI_FILTERED_10, ...mf(dur, true), ...baseArgs]);
     }
     if (!hasAudio()) {