fix: iTunes artist match and MusicBrainz first-artist-only query

iTunes: split artist by [\s,;/] not just space so "VXLLAIN, iGRES"
yields first word "vxllain" (no trailing comma) and matches correctly.

MusicBrainz: extract first artist from the full credit string (split on
,;&•) and use `artistname` field — MB stores credits like "VXLLAIN feat.
iGRES & ENXK" which never matched the comma-joined full string.

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

backend/lib/metadata.js

@@ -95,11 +95,13 @@ async function lookupItunesTrackMeta(artist, title) {
         res.on('end', () => { try { resolve(JSON.parse(d).results || []); } catch { reject(new Error('parse')); } });
       }).on('error', reject);
     });
-    const normT  = normalizeTitle(title);
-    const normA  = (artist || '').toLowerCase().trim();
+    const normT     = normalizeTitle(title);
+    const normA     = (artist || '').toLowerCase().trim();
+    // Split by space OR comma/semicolon/slash so "VXLLAIN, iGRES" → "vxllain" not "vxllain,"
+    const firstWord = s => (s || '').toLowerCase().split(/[\s,;/]/)[0];
     const match  = results.find(t =>
       normalizeTitle(t.trackName) === normT &&
-      (t.artistName || '').toLowerCase().trim().split(' ')[0] === normA.split(' ')[0]
+      firstWord(t.artistName) === firstWord(normA)
     );
     if (!match) return {};
     return {