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/lib/config.js

@@ -9,9 +9,6 @@ const CONFIG = {
   downloadBackend: (process.env.DOWNLOAD_BACKEND || 'auto').toLowerCase(),
   // squid.wtf quality: 'mp3' | 'flac' | 'hi-res'
   squidQuality:    (process.env.SQUID_QUALITY   || 'mp3').toLowerCase(),
-  // Spotify Developer credentials (Client Credentials Flow — free, no Premium needed)
-  spotifyClientId:     process.env.SPOTIFY_CLIENT_ID     || '',
-  spotifyClientSecret: process.env.SPOTIFY_CLIENT_SECRET || '',
 };
 
 module.exports = CONFIG;