fix: resolve Navidrome path mismatch across Docker containers for metadata edit

When Navidrome and the backend run in separate containers, Navidrome returns
absolute paths rooted at its own mount (e.g. /opt/navidrome/music) which
don't exist in the backend container (mounted at /music).

Add three-stage fallback:
1. _rebasePath: strip leading path components and try under CONFIG.musicPath
2. _findFileRecursive: exact filename search under CONFIG.musicPath
3. _findFileInDir: fuzzy search in album dir by track-number prefix or title stem
   (handles files renamed to include artist in the filename)

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

backend/novasonic-server.js

@@ -62,6 +62,35 @@ function _findFileRecursive(dir, name, depth = 0) {
   return null;
 }
 
+// When Navidrome runs in a different container its paths start with a different root
+// (e.g. /opt/navidrome/music) than the backend's mount (e.g. /music).
+// Strip leading components from the absolute path and try joining with CONFIG.musicPath.
+function _rebasePath(absolutePath, musicPath) {
+  const parts = absolutePath.split('/').filter(Boolean);
+  for (let i = 1; i < parts.length; i++) {
+    const candidate = path.join(musicPath, ...parts.slice(i));
+    if (fs.existsSync(candidate)) return candidate;
+  }
+  return null;
+}
+
+// When exact filename fails (e.g. file was renamed to include artist),
+// look in the same dir for any audio file sharing the track-number prefix or title stem.
+const AUDIO_EXT_RE = /\.(mp3|flac|opus|m4a|ogg|wav)$/i;
+function _findFileInDir(dir, basename) {
+  try {
+    const m      = basename.match(/^(\d+\s*-\s*)/);
+    const prefix = m ? m[1] : null;
+    const stem   = basename.replace(AUDIO_EXT_RE, '').replace(/^\d+\s*-\s*/, '').toLowerCase();
+    for (const entry of fs.readdirSync(dir)) {
+      if (!AUDIO_EXT_RE.test(entry)) continue;
+      if (prefix && entry.startsWith(prefix)) return path.join(dir, entry);
+      if (entry.replace(AUDIO_EXT_RE, '').replace(/^\d+\s*-\s*/, '').toLowerCase() === stem) return path.join(dir, entry);
+    }
+  } catch {}
+  return null;
+}
+
 // ─── QUEUE ────────────────────────────────────────────────────────────────────
 let activeDownloads = 0;
 const downloadQueue = [];
@@ -652,7 +681,9 @@ app.get('/api/song-sorttags', async (req, res) => {
   } catch (e) { return res.status(502).json({ error: e.message }); }
   if (!path.isAbsolute(filePath)) filePath = path.join(CONFIG.musicPath, filePath);
   if (!fs.existsSync(filePath)) {
-    const found = _findFileRecursive(CONFIG.musicPath, path.basename(filePath));
+    const rebased = _rebasePath(filePath, CONFIG.musicPath);
+    const found   = rebased || _findFileRecursive(CONFIG.musicPath, path.basename(filePath))
+                            || _findFileInDir(path.dirname(rebased || path.join(CONFIG.musicPath, path.basename(filePath))), path.basename(filePath));
     if (found) filePath = found; else return res.status(404).json({ error: 'File not found' });
   }
   try {
@@ -710,10 +741,17 @@ app.post('/api/edit-metadata', async (req, res) => {
   // If not absolute, try relative to music root
   if (!path.isAbsolute(filePath)) filePath = path.join(CONFIG.musicPath, filePath);
 
-  // Fallback: if Navidrome path stale/wrong, search for the file by name under music root
+  // Fallback: Navidrome may return a path from a different container mount.
+  // 1. Rebase: strip leading path components and rejoin under CONFIG.musicPath.
+  // 2. Exact recursive search under CONFIG.musicPath.
+  // 3. Fuzzy dir search: same dir, matching track-number prefix or title stem.
   if (!fs.existsSync(filePath)) {
+    const rebased  = _rebasePath(filePath, CONFIG.musicPath);
     const fileName = path.basename(filePath);
-    const found = _findFileRecursive(CONFIG.musicPath, fileName);
+    const rebasedDir = rebased ? path.dirname(rebased) : null;
+    const found = rebased
+      || _findFileRecursive(CONFIG.musicPath, fileName)
+      || (rebasedDir && _findFileInDir(rebasedDir, fileName));
     if (found) { filePath = found; }
     else return res.status(404).json({ error: `File not found: ${filePath}` });
   }