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/novasonic-server.js

@@ -17,7 +17,7 @@ const crypto         = require('crypto');
 
 const CONFIG    = require('./lib/config');
 const { jobs, updateJob, log } = require('./lib/jobs');
-const { sanitize, lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks } = require('./lib/metadata');
+const { sanitize, lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks, findExistingTrack } = require('./lib/metadata');
 const { downloadTrack, downloadAlbumTracks } = require('./lib/downloader');
 const { buildRomajiIndex, clearRomajiCache, isTokenizerReady, fetchNdSongs, romajiForStorage } = require('./lib/romaji');
 
@@ -1178,7 +1178,11 @@ app.post('/api/edit-metadata', async (req, res) => {
     const found = rebased
       || _findFileRecursive(CONFIG.musicPath, fileName)
       || _findFileInDir(rebasedDir || originalDir, fileName)
-      || _findFileByStemOrRomaji(CONFIG.musicPath, titleStem);
+      || _findFileByStemOrRomaji(CONFIG.musicPath, titleStem)
+      // Last resort: Subsonic getSong returns a *virtual* (tag-built) path, so the folder
+      // can be "[Unknown Album]" while the file actually sits in "Singles" with an artist
+      // prefix. findExistingTrack matches by normalized artist+title across all album folders.
+      || findExistingTrack(CONFIG.musicPath, ndSong.albumArtist || ndSong.artist || '', ndSong.title || '');
     if (found) { filePath = found; }
     else return res.status(404).json({ error: `File not found: ${filePath} (music root: ${CONFIG.musicPath})` });
   }