fix: pre-filter YTM search results to skip artist channel expansion

--match-filter 'webpage_url !*= /browse/' was applied to individual videos
AFTER yt-dlp expanded an artist channel into its uploads playlist — by then
each video had a /watch?v= URL, so the filter never fired.

New approach: run yt-dlp --flat-playlist --print webpage_url first to get
the raw search result URLs, filter /browse/ entries in Node.js, then pass
only the actual watch?v= video URLs to yt-dlp for download with --max-downloads 1.
The channel is never expanded; we get the real song from the search results.

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

backend/lib/ytdlp.js

@@ -8,15 +8,6 @@ const {
   downloadArtwork, fetchLyrics, findExistingTrack,
 } = require('./metadata');
 
-// Extract a plain word from an artist name for yt-dlp --match-filter uploader check.
-function artistFilterWord(name) {
-  const first = (name || '').split(/[,/•]/)[0].trim();
-  const runs = first.match(/\w+/g) || [];
-  const best = runs.filter(r => r.length >= 3).sort((a, b) => b.length - a.length)[0];
-  return best || '';
-}
-
-// Extract distinctive words from a track title for yt-dlp title ~= (?i) filters.
 function buildTitleKeywords(title) {
   const VARIANT_WORDS = new Set(['sped','speedup','slowed','reverb','nightcore','karaoke','parody','remix','cover','acoustic','instrumental']);
   const STOPWORDS = new Set(['the','a','an','and','or','of','in','on','at','to','for','feat','ft','with']);
@@ -35,7 +26,6 @@ function buildTitleKeywords(title) {
   return result;
 }
 
-// Build yt-dlp --match-filter conditions to reject variant versions.
 function buildVariantFilter(title) {
   const t = (title || '').toLowerCase();
   const RULES = [
@@ -83,11 +73,7 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     '--no-playlist',
     '--no-warnings',
     '--newline',
-    '--match-filter', 'webpage_url !*= /browse/',
   ];
-  const PI_FILTERED    = ['--playlist-items', '1-5',  '--max-downloads', '1'];
-  const PI_FILTERED_10 = ['--playlist-items', '1-10', '--max-downloads', '1'];
-  const PI_UNFILTERED  = ['--playlist-items', '1'];
 
   const spawnYtdlp = (args) => new Promise((resolve, reject) => {
     const proc = spawn(CONFIG.ytdlpBin, args, { env: { ...process.env, PYTHONUNBUFFERED: '1' } });
@@ -117,6 +103,39 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   const hasAudio = () => fs.existsSync(expectedAudio);
   const tryYtdlp = async (args) => { await spawnYtdlp(args); };
 
+  // Fetch YTM search result URLs as a flat list (no channel expansion) and return only
+  // actual video URLs (watch?v=…). This prevents yt-dlp from following an artist channel
+  // result and treating its uploads as the search result set.
+  const getYtmVideoUrls = (maxItems) => new Promise(resolve => {
+    const urls = [];
+    const proc = spawn(CONFIG.ytdlpBin, [
+      ytmUrl,
+      '--flat-playlist',
+      '--playlist-items', `1-${maxItems}`,
+      '--print', 'webpage_url',
+      '--no-warnings',
+    ], { env: { ...process.env, PYTHONUNBUFFERED: '1' } });
+    proc.stdout.on('data', d => {
+      d.toString().split('\n').forEach(line => {
+        const u = line.trim();
+        // Only accept video URLs — skip /browse/ (artist channel) and /playlist? entries
+        if (u.includes('watch?v=')) urls.push(u);
+      });
+    });
+    proc.stderr.on('data', () => {});
+    proc.on('error', () => resolve([]));
+    proc.on('close', () => resolve(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 tryYtdlp([...urls, '--max-downloads', '1', ...mfArgs, ...baseArgs]);
+  };
+
   const variantFilter = buildVariantFilter(title);
   const titleKws      = buildTitleKeywords(title);
   const variantPart   = variantFilter ? ` & ${variantFilter}` : '';
@@ -128,63 +147,83 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
       `${durPart}!is_live${titleKwPart}${variant ? variantPart : ''}`];
   };
 
+  const PI_FILTERED    = ['--playlist-items', '1-5',  '--max-downloads', '1'];
+  const PI_FILTERED_10 = ['--playlist-items', '1-10', '--max-downloads', '1'];
+  const PI_UNFILTERED  = ['--playlist-items', '1'];
+
   if (durationMs) {
     const durationSec = Math.round(durationMs / 1000);
     const margin = Math.max(15, Math.round(durationSec * 0.15));
     const dur = { min: Math.max(30, durationSec - margin), max: durationSec + margin };
 
     log(jobId, `🔍 Suche YT Music (~${durationSec}s ±${margin}s): "${query}"`);
-    await tryYtdlp([ytmUrl, ...PI_FILTERED, ...mf(dur, true), ...baseArgs]);
+    // 1. YTM top-5 — pre-filtered URLs (no channel expansion), duration + title + variant filter
+    await tryYtmSearch(5, mf(dur, true));
     if (!hasAudio()) {
+      // 2. YT top-5 — ytsearch always returns videos (no channels), safe to use directly
       log(jobId, '⚠️  Kein YTM-Treffer — Fallback zu YouTube…');
       await tryYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(dur, true), ...baseArgs]);
     }
     if (!hasAudio()) {
+      // 3. YTM top-10 — wider net
       log(jobId, '⚠️  Erweitere Suche auf top 10 mit Dauerbeschränkung…');
-      await tryYtdlp([ytmUrl, ...PI_FILTERED_10, ...mf(dur, true), ...baseArgs]);
+      await tryYtmSearch(10, mf(dur, true));
     }
     if (!hasAudio()) {
+      // 4. YT top-10
       await tryYtdlp([`ytsearch10:${query}`, ...PI_FILTERED_10, ...mf(dur, true), ...baseArgs]);
     }
     if (!hasAudio()) {
+      // 5. YTM top-5 — no duration check
       log(jobId, '⚠️  Kein Treffer im Dauerbereich — suche ohne Dauerbeschränkung…');
-      await tryYtdlp([ytmUrl, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
+      await tryYtmSearch(5, mf(null, true));
     }
     if (!hasAudio()) {
+      // 6. YT top-5 — no duration check
       await tryYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
     }
     if (!hasAudio()) {
+      // 7. YTM top-5 — title only (no variant filter)
       log(jobId, '⚠️  Letzter gefilterter Versuch…');
-      await tryYtdlp([ytmUrl, ...PI_FILTERED, ...mf(null, false), ...baseArgs]);
+      await tryYtmSearch(5, mf(null, false));
     }
     if (!hasAudio()) {
+      // 8. YT top-5 — title only
       await tryYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, false), ...baseArgs]);
     }
     if (!hasAudio()) {
+      // 9. YTM — no filter at all, take first video result
       log(jobId, '⚠️  Letzter Versuch: keine Filter…');
-      await tryYtdlp([ytmUrl, ...PI_UNFILTERED, ...baseArgs]);
+      await tryYtmSearch(3, []);
     }
     if (!hasAudio()) {
+      // 10. YT — no filter
       await tryYtdlp([`ytsearch1:${query}`, ...PI_UNFILTERED, ...baseArgs]);
     }
   } else {
     log(jobId, `🔍 Suche YT Music: "${query}"`);
-    await tryYtdlp([ytmUrl, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
+    // 1. YTM top-5 — pre-filtered, title + variant
+    await tryYtmSearch(5, mf(null, true));
     if (!hasAudio()) {
+      // 2. YT top-5
       await tryYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, true), ...baseArgs]);
     }
     if (!hasAudio()) {
+      // 3. YTM top-5 — title only
       log(jobId, '⚠️  Letzter gefilterter Versuch…');
-      await tryYtdlp([ytmUrl, ...PI_FILTERED, ...mf(null, false), ...baseArgs]);
+      await tryYtmSearch(5, mf(null, false));
     }
     if (!hasAudio()) {
+      // 4. YT top-5 — title only
       await tryYtdlp([`ytsearch5:${query}`, ...PI_FILTERED, ...mf(null, false), ...baseArgs]);
     }
     if (!hasAudio()) {
+      // 5. YTM — first video result, no filter
       log(jobId, '⚠️  Letzter Versuch: keine Filter…');
-      await tryYtdlp([ytmUrl, ...PI_UNFILTERED, ...baseArgs]);
+      await tryYtmSearch(3, []);
     }
     if (!hasAudio()) {
+      // 6. YT — no filter
       await tryYtdlp([`ytsearch1:${query}`, ...PI_UNFILTERED, ...baseArgs]);
     }
   }
@@ -195,7 +234,7 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     return { success: true, dir };
   }
 
-  return embedMetadata({ jobId, audioFile, dir, trackPrefix, safeTitle, artist, mainArtist, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, title, lyricsArtist: artist });
+  return embedMetadata({ jobId, audioFile, dir, trackPrefix, safeTitle, artist, mainArtist, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, title });
 }
 
 // Shared ffmpeg metadata embedding — used by both yt-dlp and squid paths