fix(backend): keep source artist name as-is, no iTunes override

Artist name from Spotify/source is stored directly in folder, filename,
and tags without being replaced by the iTunes artistName. iTunes is used
only for supplemental metadata (genre, year, cover, duration, trackNum,
albumArtist/collectionArtistName). Sort tags still receive romaji via
romajiForStorage() in ytdlp.js.

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

backend/novasonic-server.js

@@ -600,8 +600,8 @@ app.post('/api/download', async (req, res) => {
             if (!meta.trackNum    && genreMeta.trackNum)     { meta.trackNum    = genreMeta.trackNum;    }
           }
         }
-        const finalArtist = meta.itunesArtistName || artist;
-        const result = await downloadTrack({ jobId, artist: finalArtist, rawArtist: artist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
+        // Keep source (Spotify) artist name as-is — no iTunes override
+        const result = await downloadTrack({ jobId, artist, rawArtist: artist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
         if (result?.skipped) {
           updateJob(jobId, { status: 'done', progress: 100, message: 'Bereits vorhanden – übersprungen', skipped: true });
         } else if (result?.notFound) {
@@ -688,14 +688,13 @@ app.post('/api/download', async (req, res) => {
               log(jobId, `📋 iTunes: "${trackArtist} – ${trackTitle}"`);
               itunes = await lookupItunesTrackMeta(searchArtist, itunesTitle);
             }
-            // Prefer iTunes-resolved name (from server lookup or frontend enrichment) for sort tags / folders.
-            // trackArtist stays as rawArtist for main tags and YTM search.
-            const resolvedArtist = itunes.itunesArtistName || track.itunesArtistName || trackArtist;
+            // Keep source (Spotify) artist name as-is — no iTunes override.
+            // iTunes collectionArtistName is used as albumArtist for consistent album grouping.
             return downloadTrack({
               jobId,
-              artist:      resolvedArtist, // iTunes English name → used for sort tags + folder names
-              rawArtist:   trackArtist,    // original Spotify/Japanese name → used for main tag + YTM search
-              albumArtist: resolvedArtist,
+              artist:      trackArtist,
+              rawArtist:   trackArtist,
+              albumArtist: itunes.albumArtist || trackArtist,
               title:       trackTitle,
               album:       itunes.album       || track.collectionName || track.album || '',
               year:        itunes.year        || (track.releaseDate ? new Date(track.releaseDate).getFullYear() : undefined) || track.year,