remove: lyrics downloading (fetchLyrics, .lrc files, uslt tag embedding)

Removes LRCLIB lookup, .lrc sidecar file writing, and lyrics/uslt
metadata embedding from all downloads.

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

backend/lib/metadata.js

@@ -158,5 +158,5 @@ async function searchItunesAlbumTracks(artist, album) {
 module.exports = {
   sanitize, navidromeArtist,
   downloadArtwork, findExistingTrack,
-  fetchLyrics, lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks,
+  lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks,
 };