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/novasonic-server.js

@@ -627,7 +627,11 @@ app.post('/api/download', async (req, res) => {
         // over the request's `artist`, which can be a single track's performer — e.g. the lead single
         // "TAKEDOWN" is by TWICE, so `artist` arrives as "TWICE" and would tag the whole soundtrack's
         // album_artist as TWICE. Keeps album_artist consistent so Navidrome doesn't split the album.
-        const albumArtistName = tracks.map(t => t.collectionArtistName).find(Boolean) || artist;
+        let albumArtistName = tracks.map(t => t.collectionArtistName).find(Boolean) || artist;
+        // The JP store localizes Western artists to katakana ("DJスネイク"). If iTunes' album artist is
+        // Japanese but the request artist isn't, keep the request's English name (tag + folder).
+        const JP_RE = /[぀-ヿ㐀-䶿一-鿿]/;
+        if (albumArtistName && artist && JP_RE.test(albumArtistName) && !JP_RE.test(artist)) albumArtistName = artist;
         if (albumArtistName !== artist) log(jobId, `👥 Album-Künstler: "${albumArtistName}" (iTunes)`);
         const { done: dlDone, skipped: dlSkipped } = await downloadAlbumTracks({ jobId, artist: albumArtistName, albumTitle: album || title, tracks, artworkUrl });
         const allSkipped  = dlSkipped === tracks.length;
@@ -655,7 +659,10 @@ app.post('/api/download', async (req, res) => {
           if (tracks.length) {
             // Same as the album path: album_artist from iTunes' album-level artist, so a collaboration
             // or soundtrack in the artist's discography isn't tagged with the searched artist.
-            const albArtist = tracks.map(t => t.collectionArtistName).find(Boolean) || alb.artistName || artist;
+            let albArtist = tracks.map(t => t.collectionArtistName).find(Boolean) || alb.artistName || artist;
+            // Don't let the JP store's katakana album-artist override a non-Japanese searched artist.
+            const JP_RE = /[぀-ヿ㐀-䶿一-鿿]/;
+            if (albArtist && artist && JP_RE.test(albArtist) && !JP_RE.test(artist)) albArtist = artist;
             await downloadAlbumTracks({ jobId, artist: albArtist, albumTitle: alb.collectionName, tracks, artworkUrl: alb.artworkUrl100 });
           }
           done++;