fix(backend): stricter duration filter + variant title filter

- Duration: ±15% (min 15s) instead of -40%/+50%
- Variant filter: reject sped/speedup/slowed/reverb/nightcore/karaoke/
  parody/remix/cover/acoustic/instrumental unless original title has keyword
- Cascade: YTM+YT tried at each level (dur+artist+variant → no artist → no dur → no filter)

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

backend/novasonic-server.js

@@ -65,6 +65,27 @@ function artistFilterWord(name) {
   return word.length >= 3 ? word : '';
 }
 
+// Build yt-dlp --match-filter conditions to reject variant versions (speed up, slowed, remix…)
+// unless the original track title already contains those keywords.
+function buildVariantFilter(title) {
+  const t = (title || '').toLowerCase();
+  const RULES = [
+    { key: 'sped',         expr: 'title !*= sped' },         // "sped up"
+    { key: 'speedup',      expr: 'title !*= speedup' },      // "speedup"
+    { key: 'slowed',       expr: 'title !*= slowed' },
+    { key: 'reverb',       expr: 'title !*= reverb' },
+    { key: 'nightcore',    expr: 'title !*= nightcore' },
+    { key: 'karaoke',      expr: 'title !*= karaoke' },
+    { key: 'parody',       expr: 'title !*= parody' },
+    { key: 'remix',        expr: 'title !*= remix' },
+    { key: 'cover',        expr: 'title !*= cover' },
+    { key: 'acoustic',     expr: 'title !*= acoustic' },
+    { key: 'instrumental', expr: 'title !*= instrumental' },
+  ];
+  const parts = RULES.filter(r => !t.includes(r.key)).map(r => r.expr);
+  return parts.length ? parts.join(' & ') : '';
+}
+
 // Convert multi-artist strings to " / " separator so Navidrome splits correctly.
 // Handles: "feat." / "ft." / "featuring" and comma-separated artists from Spotify/iTunes.
 function navidromeArtist(name) {
@@ -227,32 +248,54 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   const hasAudio = () => fs.readdirSync(dir).some(f => AUDIO_EXTS.some(e => f.endsWith(`.${e}`)));
   const ytmUrl = `https://music.youtube.com/search?q=${encodeURIComponent(query)}`;
 
-  const uploaderWord = artistFilterWord(mainArtist);
+  const uploaderWord   = artistFilterWord(mainArtist);
-  const uploaderFilter = uploaderWord ? ` & uploader *= ${uploaderWord}` : '';
+  const variantFilter  = buildVariantFilter(title);
+  const uploaderPart   = uploaderWord  ? ` & uploader *= ${uploaderWord}` : '';
+  const variantPart    = variantFilter ? ` & ${variantFilter}` : '';
+
+  // Build a --match-filter arg from active parts (duration, artist, variant).
+  // !is_live is always included.
+  const mf = (dur, artist, variant) => {
+    const durPart = dur ? `duration > ${dur.min} & duration < ${dur.max} & ` : '';
+    return ['--match-filter',
+      `${durPart}!is_live${artist ? uploaderPart : ''}${variant ? variantPart : ''}`];
+  };
 
   if (durationMs) {
     const durationSec = Math.round(durationMs / 1000);
-    const minDur = Math.max(30, Math.round(durationSec * 0.6));
+    const margin = Math.max(15, Math.round(durationSec * 0.15)); // ±15 % (min 15 s)
-    const maxDur = Math.round(durationSec * 1.5);
+    const dur = { min: Math.max(30, durationSec - margin), max: durationSec + margin };
-    const durArtistFilter = ['--match-filter', `duration > ${minDur} & duration < ${maxDur} & !is_live${uploaderFilter}`];
-    const durOnlyFilter   = ['--match-filter', `duration > ${minDur} & duration < ${maxDur} & !is_live`];
 
-    log(jobId, `🔍 Suche YT Music (~${durationSec}s): "${query}"`);
+    log(jobId, `🔍 Suche YT Music (~${durationSec}s ±${margin}s): "${query}"`);
-    await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...durArtistFilter, ...baseArgs]);
+    // 1. YTM — Dauer + Artist + Varianten-Filter (streng)
+    await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...mf(dur, true, true), ...baseArgs]);
 
     if (!hasAudio()) {
-      log(jobId, '⚠️  Kein YT-Music-Treffer (Dauer+Artist) — Fallback zu YouTube…');
+      // 2. YT — Dauer + Artist + Varianten-Filter
-      await spawnYtdlp([`ytsearch1:${query}`, ...durArtistFilter, ...baseArgs]);
+      log(jobId, '⚠️  Kein YTM-Treffer — Fallback zu YouTube…');
+      await spawnYtdlp([`ytsearch1:${query}`, ...mf(dur, true, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      log(jobId, '⚠️  Kein Treffer mit Artist-Filter — Suche nur nach Dauer…');
+      // 3. YTM — Dauer + Varianten-Filter (kein Artist)
-      await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...durOnlyFilter, ...baseArgs]);
+      log(jobId, '⚠️  Kein Treffer mit Artist-Filter — ohne Artist…');
+      await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...mf(dur, false, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      await spawnYtdlp([`ytsearch1:${query}`, ...durOnlyFilter, ...baseArgs]);
+      // 4. YT — Dauer + Varianten-Filter
+      await spawnYtdlp([`ytsearch1:${query}`, ...mf(dur, false, true), ...baseArgs]);
     }
     if (!hasAudio()) {
-      log(jobId, '⚠️  Kein Treffer im Dauerbereich — Suche ohne Filter…');
+      // 5. YTM — nur Varianten-Filter (kein Dauer-Check)
+      log(jobId, '⚠️  Kein Treffer im Dauerbereich — suche ohne Dauerbeschränkung…');
+      await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...mf(null, false, true), ...baseArgs]);
+    }
+    if (!hasAudio()) {
+      // 6. YT — nur Varianten-Filter
+      await spawnYtdlp([`ytsearch1:${query}`, ...mf(null, false, true), ...baseArgs]);
+    }
+    if (!hasAudio()) {
+      // 7+8. Letzter Ausweg: komplett ungefiltert
+      log(jobId, '⚠️  Letzter Versuch: keine Filter…');
       await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...baseArgs]);
     }
     if (!hasAudio()) {
@@ -260,14 +303,21 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     }
   } else {
     log(jobId, `🔍 Suche YT Music: "${query}"`);
-    await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...(uploaderFilter ? ['--match-filter', `!is_live${uploaderFilter}`] : []), ...baseArgs]);
+    // Ohne Dauer: Artist + Varianten → ohne Artist → ohne alles
+    await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...mf(null, true, true), ...baseArgs]);
 
     if (!hasAudio()) {
-      log(jobId, '⚠️  Kein YT-Music-Treffer — Fallback zu YouTube…');
+      await spawnYtdlp([`ytsearch1:${query}`, ...mf(null, true, true), ...baseArgs]);
-      await spawnYtdlp([`ytsearch1:${query}`, ...(uploaderFilter ? ['--match-filter', `!is_live${uploaderFilter}`] : []), ...baseArgs]);
     }
-    if (!hasAudio() && uploaderFilter) {
+    if (!hasAudio()) {
-      log(jobId, '⚠️  Kein Treffer mit Artist-Filter — Suche ohne…');
+      log(jobId, '⚠️  Kein Treffer mit Artist — ohne Artist-Filter…');
+      await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...mf(null, false, true), ...baseArgs]);
+    }
+    if (!hasAudio()) {
+      await spawnYtdlp([`ytsearch1:${query}`, ...mf(null, false, true), ...baseArgs]);
+    }
+    if (!hasAudio()) {
+      log(jobId, '⚠️  Letzter Versuch: keine Filter…');
       await spawnYtdlp([ytmUrl, '--playlist-items', '1', ...baseArgs]);
     }
     if (!hasAudio()) {