feat: accept space, &, / as artist separators in metadata editor

- Space as fallback separator on save when no explicit separator present
  (Artist1 Artist2 → Artist1;Artist2)
- & and / added to explicit separator set everywhere
- navidromeArtist (download path) also handles /

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

backend/lib/metadata.js

@@ -13,6 +13,7 @@ function navidromeArtist(name) {
     .replace(/\s+ft\.?\s+/gi, ';')
     .replace(/\s+featuring\s+/gi, ';')
     .replace(/\s+&\s+/g, ';')
+    .replace(/\s*\/\s*/g, ';')
     .replace(/,\s*/g, ';');
 }