feat: use ; as artist separator in downloads, add & handling

navidromeArtist() now writes ; instead of / so Navidrome splits
correctly with the updated Tags.Artist.Split config.
Also handles & (e.g. "Artist A & Artist B" → "Artist A;Artist B").

Add fix-artist-separators.sh — one-time Debian script to update
existing library files from " / " to ";" in artist tags.

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

backend/lib/metadata.js

@@ -9,10 +9,11 @@ function sanitize(s) {
 function navidromeArtist(name) {
   if (!name) return name;
   return name
-    .replace(/\s+feat\.?\s+/gi, ' / ')
-    .replace(/\s+ft\.?\s+/gi, ' / ')
-    .replace(/\s+featuring\s+/gi, ' / ')
-    .replace(/,\s+/g, ' / ');
+    .replace(/\s+feat\.?\s+/gi, ';')
+    .replace(/\s+ft\.?\s+/gi, ';')
+    .replace(/\s+featuring\s+/gi, ';')
+    .replace(/\s+&\s+/g, ';')
+    .replace(/,\s+/g, ';');
 }
 
 // Download hi-res iTunes artwork (1000x1000) to cover.jpg, return path or null