fix: restore playlist membership after metadata edit renames/moves file

When a file is renamed or moved to a new artist/album folder during
metadata edit, Navidrome assigns a new song ID, breaking all playlist
entries that referenced the old ID.

Before the edit: collect which playlists contain the song.
After rename + rescan: poll Navidrome until the new song appears,
then update each affected playlist (add new ID, remove old index).

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

backend/novasonic-server.js

@@ -724,6 +724,64 @@ app.get('/api/song-sorttags', async (req, res) => {
   } catch (e) { res.status(500).json({ error: e.message }); }
 });
 
+// ─── PLAYLIST MIGRATION HELPERS ──────────────────────────────────────────────
+// Used to preserve playlist membership when a file is renamed/moved during edit.
+
+async function _ndApi(ndUrl, user, pass, method, extra = '') {
+  const s = Math.random().toString(36).slice(2, 10);
+  const t = crypto.createHash('md5').update(pass + s).digest('hex');
+  const r = await fetch(
+    `${ndUrl}/rest/${method}?u=${encodeURIComponent(user)}&t=${t}&s=${s}&v=1.16.1&c=novasonic&f=json${extra}`,
+    { signal: AbortSignal.timeout(8000) }
+  );
+  return r.json();
+}
+
+async function _playlistsContaining(ndUrl, user, pass, songId) {
+  try {
+    const d = await _ndApi(ndUrl, user, pass, 'getPlaylists');
+    const pls = [].concat(d?.['subsonic-response']?.playlists?.playlist || []);
+    const affected = [];
+    for (const pl of pls) {
+      const det = await _ndApi(ndUrl, user, pass, 'getPlaylist', `&id=${pl.id}`);
+      const entries = [].concat(det?.['subsonic-response']?.playlist?.entry || []);
+      if (entries.some(e => e.id === songId)) affected.push(pl.id);
+    }
+    return affected;
+  } catch { return []; }
+}
+
+// Poll Navidrome until the renamed song appears (up to ~5s)
+async function _findSongAfterRename(ndUrl, user, pass, title, artist, oldId) {
+  for (let i = 0; i < 5; i++) {
+    if (i > 0) await new Promise(r => setTimeout(r, 1200));
+    try {
+      const q   = encodeURIComponent(`${artist} ${title}`.trim().slice(0, 100));
+      const d   = await _ndApi(ndUrl, user, pass, 'search3', `&query=${q}&songCount=20&artistCount=0&albumCount=0`);
+      const songs = [].concat(d?.['subsonic-response']?.searchResult3?.song || []);
+      const match = songs.find(s =>
+        s.id !== oldId &&
+        (s.title || '').toLowerCase() === (title || '').toLowerCase()
+      );
+      if (match) return match;
+    } catch {}
+  }
+  return null;
+}
+
+async function _migratePlaylists(ndUrl, user, pass, plIds, oldId, newId) {
+  for (const plId of plIds) {
+    try {
+      const det     = await _ndApi(ndUrl, user, pass, 'getPlaylist', `&id=${plId}`);
+      const entries = [].concat(det?.['subsonic-response']?.playlist?.entry || []);
+      const idx     = entries.findIndex(e => e.id === oldId);
+      let extra = `&songIdToAdd=${newId}`;
+      if (idx >= 0) extra += `&songIndexToRemove=${idx}`;
+      await _ndApi(ndUrl, user, pass, 'updatePlaylist', `&playlistId=${plId}${extra}`);
+    } catch {}
+  }
+}
+
 // ─── METADATA EDIT ───────────────────────────────────────────────────────────
 app.post('/api/edit-metadata', async (req, res) => {
   const { ndUrl, user, pass, songId, title, artist, albumArtist, album, year, trackNum, totalTracks,
@@ -757,6 +815,10 @@ app.post('/api/edit-metadata', async (req, res) => {
     return res.status(502).json({ error: `Navidrome lookup failed: ${e.message}` });
   }
 
+  // Collect playlists containing this song BEFORE the edit so we can restore membership
+  // if the file gets renamed/moved (which changes the Navidrome song ID).
+  const affectedPlaylists = await _playlistsContaining(ndUrl, user, pass, songId).catch(() => []);
+
   // If not absolute, try relative to music root
   if (!path.isAbsolute(filePath)) filePath = path.join(CONFIG.musicPath, filePath);
 
@@ -910,6 +972,16 @@ app.post('/api/edit-metadata', async (req, res) => {
       await fetch(`${ndUrl}/rest/startScan?u=${encodeURIComponent(user)}&t=${tok2}&s=${salt2}&v=1.16.1&c=novasonic&f=json`, { signal: AbortSignal.timeout(5000) });
     } catch {}
 
+    // Restore playlist membership after rename/move — the rescan gives the renamed song a new ID
+    if (affectedPlaylists.length > 0) {
+      const effectiveTitle  = title  || ndSong?.title  || '';
+      const effectiveArtist = (artist || ndSong?.artist || '').split(';')[0].trim();
+      const newSong = await _findSongAfterRename(ndUrl, user, pass, effectiveTitle, effectiveArtist, songId);
+      if (newSong && newSong.id !== songId) {
+        await _migratePlaylists(ndUrl, user, pass, affectedPlaylists, songId, newSong.id);
+      }
+    }
+
     res.json({ ok: true });
   } catch (e) {
     try { if (fs.existsSync(tmpFile)) fs.unlinkSync(tmpFile); } catch {}