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/metadata.js

@@ -138,6 +138,6 @@ async function searchItunesAlbumTracks(artist, album) {
 
 module.exports = {
   sanitize, navidromeArtist,
-  downloadArtwork, AUDIO_EXTS, findExistingTrack,
+  downloadArtwork, findExistingTrack,
   fetchLyrics, lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks,
 };