feat(0.9): romaji sort-tags only, robust dupe detection, Mittelweg no-duration, autofill all fields

- Japanese tracks stored with original names in folder/filename/tags; romaji only in sort tags
- findExistingTrack: case-insensitive, searches all album folders under artist, strips release suffix
- --max-filesize 30M + 2x duration cap on all yt-dlp calls
- iTunes collectionArtistName captured as album_artist; fixes compilation album splitting
- album_artist tag uses mainArtist (not rawArtist) so all tracks in one album are consistent
- No-duration cascade: steps 5-8 (title-only/no-filter) removed — fail cleanly like with-duration
- Autofill iTunes: fills title, artist, albumArtist, discNum (was album/genre/year/track/total only)
- Autofill MusicBrainz: fills albumArtist, discNum (was missing both)

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

backend/lib/metadata.js

@@ -48,13 +48,50 @@ async function downloadArtwork(url, dir) {
 
 const AUDIO_EXTS = ['mp3', 'flac', 'opus', 'm4a', 'ogg', 'wav'];
 
-function findExistingTrack(dir, trackPrefix, safeTitle, safeArtist) {
-  // Check new format "{prefix}{artist} - {title}.ext" first, then old format "{prefix}{title}.ext"
-  const names = safeArtist
-    ? [`${trackPrefix}${safeArtist} - ${safeTitle}`, `${trackPrefix}${safeTitle}`]
-    : [`${trackPrefix}${safeTitle}`];
-  return AUDIO_EXTS.flatMap(ext => names.map(n => path.join(dir, `${n}.${ext}`)))
-    .find(f => fs.existsSync(f)) || null;
+// Strips release-type suffixes like "- Single", "- EP", "2024" from end of title/stem.
+const _RELEASE_SUFFIX_RE = /\s*[-–]\s*(?:(?:official\s+)?(?:music\s+)?(?:single|ep|lp|album|ost|soundtrack|remaster(?:ed)?|deluxe(?:\s+edition)?|bonus\s+tracks?|b-?sides?|version|edit)\b.*|(?:20|19)\d{2})\s*$/i;
+
+// Robust duplicate detection: search case-insensitively across all album folders
+// under the artist folder matching firstArtist, compare normalized title ignoring
+// release suffixes and track prefixes. Handles DJ OKAWARI vs DJ Okawari, and
+// "recognize - Single.mp3" vs "recognize.mp3" (drift between runs).
+function findExistingTrack(musicRoot, firstArtist, cleanTitle) {
+  const norm = s => (s || '').toLowerCase()
+    .replace(/[\[(（][^\]）)]*[\]）)]/g, '')
+    .replace(/[,.'!?]/g, '')
+    .replace(/\s+/g, ' ')
+    .trim();
+  const normArtist = norm(firstArtist);
+  const normTitle  = norm(cleanTitle);
+  // Accept both the raw cleanTitle and version without release suffix (e.g. "recognize" matches "recognize - Single")
+  const titleVariants = new Set([normTitle, norm(cleanTitle.replace(_RELEASE_SUFFIX_RE, ''))]);
+
+  try {
+    for (const ad of fs.readdirSync(musicRoot, { withFileTypes: true })) {
+      if (!ad.isDirectory() || norm(ad.name) !== normArtist) continue;
+      const artistPath = path.join(musicRoot, ad.name);
+      try {
+        for (const abd of fs.readdirSync(artistPath, { withFileTypes: true })) {
+          if (!abd.isDirectory()) continue;
+          const albumPath = path.join(artistPath, abd.name);
+          try {
+            for (const f of fs.readdirSync(albumPath)) {
+              const ext = path.extname(f).slice(1).toLowerCase();
+              if (!AUDIO_EXTS.includes(ext)) continue;
+              // Strip extension + leading track prefix ("01 - ", "01-15 – " etc.)
+              const stem = f.slice(0, -(ext.length + 1)).replace(/^\d[\d-]*\s*[-–—]\s*/, '');
+              // Stem may be "Artist - Title" or just "Title"
+              const di = stem.indexOf(' - ');
+              const rawTitle = di >= 0 ? stem.slice(di + 3) : stem;
+              const normStemTitle = norm(rawTitle.replace(_RELEASE_SUFFIX_RE, ''));
+              if (titleVariants.has(normStemTitle)) return path.join(albumPath, f);
+            }
+          } catch {}
+        }
+      } catch {}
+    }
+  } catch {}
+  return null;
 }
 
 // Fetch plain + synced lyrics from LRCLIB
@@ -140,7 +177,9 @@ async function lookupItunesTrackMeta(artist, title) {
     const rawGenre = (usMatch || match).primaryGenreName;
     const genre = rawGenre ? toEnglishGenre(rawGenre) : undefined;
     return {
-      album:            match.collectionName || '',
+      album:            match.collectionName      || '',
+      albumArtist:      match.collectionArtistName || undefined,
+      trackName:        match.trackName            || undefined,
       year:             match.releaseDate ? new Date(match.releaseDate).getFullYear() : undefined,
       trackNum:         match.trackNumber   || undefined,
       totalTracks:      match.trackCount    || undefined,