fix(backend): consistent album for no-album downloads + tolerant editor resolve

Two-part fix for "[Unknown Album]" vs "Singles" mismatch that broke the
metadata editor (File not found):

- Download: always write an album tag, defaulting to "Singles" so it matches
  the on-disk folder (album || 'Singles'). No-album tracks no longer show as
  "[Unknown Album]" while living in a "Singles" folder. (Future downloads.)
- Editor resolve: Subsonic getSong returns a *virtual* tag-built path (e.g.
  "[Unknown Album]/Title.mp3" with raw "|"), not the physical file ("Singles/
  Artist - Title_YTPMV.mp3"). Add findExistingTrack as a final fallback, and
  make its normalizer map the sanitized chars (/ \ : * ? " < > | and _) to a
  space so a title tag matches the sanitized filename. (Fixes existing tracks.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/lib/metadata.js

@@ -58,7 +58,10 @@ const _RELEASE_SUFFIX_RE = /\s*[-–]\s*(?:(?:official\s+)?(?:music\s+)?(?:singl
 function findExistingTrack(musicRoot, firstArtist, cleanTitle) {
   const norm = s => (s || '').toLowerCase()
     .replace(/[\[(（][^\]）)]*[\]）)]/g, '')
-    .replace(/[,.'!?]/g, '')
+    // Filesystem-sanitized chars (/ \ : * ? " < > |) become "_" on disk — map those AND "_"
+    // to a space so a title tag "Kwik Flip | YTPMV" matches the file "Kwik Flip _ YTPMV".
+    .replace(/[/\\:*?"<>|_]/g, ' ')
+    .replace(/[,.'!]/g, '')
     .replace(/\s+/g, ' ')
     .trim();
   const normArtist = norm(firstArtist);