refactor: remove dead code from backend and lib

- Remove unused spotifyClientId/spotifyClientSecret from config
- Remove AUDIO_EXTS from metadata exports (internal-only)
- Remove tryYtdlp wrapper in ytdlp.js (direct spawnYtdlp calls)
- Remove _spArtistName wrapper in server (direct _spAllArtists calls)
- Fix stale comment: primary resolver is SpotipyFree, not curl_cffi

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

backend/novasonic-server.js

@@ -296,10 +296,6 @@ function _spAllArtists(artistsRaw) {
   return arr.map(a => a?.profile?.name || a?.name || '').filter(Boolean).join(', ');
 }
 
-function _spArtistName(artistsRaw) {
-  return _spAllArtists(artistsRaw);
-}
-
 function _spTrackItem(t, defaultAlbum, defaultArt, defaultAlbumArtist) {
   const trackName = t?.name || '';
   const artist    = _spAllArtists(t?.artists) || defaultAlbumArtist || '';
@@ -377,7 +373,7 @@ async function _resolveSpotifyViaSpotdl(url) {
 }
 
 async function _resolveSpotify(type, id, originalUrl) {
-  // Primary: curl_cffi Chrome-impersonation + TOTP (no rate limit bucket as api keys)
+  // Primary: SpotipyFree (SpotAPI + tls_client Chrome fingerprint, Spotify internal endpoints)
   try {
     console.log('[spotify] trying spotdl resolver');
     return await _resolveSpotifyViaSpotdl(originalUrl);
@@ -391,7 +387,7 @@ async function _resolveSpotify(type, id, originalUrl) {
 
   if (type === 'track') {
     const t = await _spApi(`/tracks/${id}`, token);
-    const artist = _spArtistName(t.artists);
+    const artist = _spAllArtists(t.artists);
     const artUrl = _spArtwork(t);
     return { type: 'track', name: t.name || '', artist, artworkUrl: artUrl,
       tracks: [_spTrackItem(t, t.album?.name, artUrl, artist)] };
@@ -399,7 +395,7 @@ async function _resolveSpotify(type, id, originalUrl) {
 
   if (type === 'album') {
     const album = await _spApi(`/albums/${id}`, token);
-    const artist = _spArtistName(album.artists);
+    const artist = _spAllArtists(album.artists);
     const artUrl = _spArtwork(album);
     const trackItems = await _spAllAlbumTracks(id, token);
     return { type: 'album', name: album.name || '', artist, artworkUrl: artUrl,