fix(backend): don't tag Western artists with the JP store's katakana album-artist

The iTunes lookup is JP-first (for correct Japanese titles/artwork), but the Japanese
store returns katakana-localized names for Western artists — so DJ Snake's single got
album_artist / folder "DJスネイク • アルーナジョージ" while the title/artist stayed English.

- lookupItunesTrackMeta: take the NAMES (album, album-artist, track, artist) from the US
  store when the search isn't Japanese; keep JP names for actual Japanese songs.
- album & artist download paths: if iTunes' collectionArtistName is Japanese but the
  request artist isn't, keep the request's English name for the tag and folder.

Already-downloaded files aren't rewritten — re-download or fix via the editor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/lib/metadata.js

@@ -179,22 +179,27 @@ async function lookupItunesTrackMeta(artist, title) {
     let usMatch = strictMatch(usResults) || (!isJpSearch && titleOnlyMatch(usResults));
     if (!match) match = usMatch;
     if (!match) return {};
+    // For non-Japanese songs, take the NAMES from the US store: the JP store returns katakana-
+    // localized artist / album-artist names for Western artists (e.g. "DJスネイク" for DJ Snake),
+    // which would otherwise become the album_artist tag and the on-disk folder. Japanese songs keep
+    // the JP names (correct kanji). Numeric fields (year, track#, duration) are store-agnostic.
+    const naming = (!isJpSearch && usMatch) ? usMatch : match;
     // JP genre names are localized (e.g. イージーリスニング); prefer US genre, map JP→EN as fallback
     const rawGenre = (usMatch || match).primaryGenreName;
     const genre = rawGenre ? toEnglishGenre(rawGenre) : undefined;
     return {
-      album:            match.collectionName      || '',
-      albumArtist:      match.collectionArtistName || undefined,
-      trackName:        match.trackName            || undefined,
+      album:            naming.collectionName       || '',
+      albumArtist:      naming.collectionArtistName  || undefined,
+      trackName:        naming.trackName             || undefined,
       year:             match.releaseDate ? new Date(match.releaseDate).getFullYear() : undefined,
       trackNum:         match.trackNumber   || undefined,
       totalTracks:      match.trackCount    || undefined,
       discNum:          match.discNumber    || undefined,
       discTotal:        match.discCount     || undefined,
       genre:            genre,
-      artworkUrl:       (match.artworkUrl100 || '').replace('100x100bb', '1000x1000bb'),
+      artworkUrl:       (naming.artworkUrl100 || match.artworkUrl100 || '').replace('100x100bb', '1000x1000bb'),
       durationMs:       match.trackTimeMillis  || undefined,
-      itunesArtistName: match.artistName       || undefined,
+      itunesArtistName: naming.artistName            || undefined,
     };
   } catch { return {}; }
 }