feat: server-side romaji overrides (shared list); preload ExoPlayer from SharedPreferences on cold start

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

backend/lib/config.js

@@ -1,6 +1,7 @@
 const CONFIG = {
   port:            process.env.PORT             || 3700,
   musicPath:       process.env.MUSIC_PATH       || '/music',
+  dataPath:        process.env.DATA_PATH        || '/data',
   ytdlpBin:        process.env.YTDLP_BIN       || 'yt-dlp',
   audioFmt:        process.env.AUDIO_FMT       || 'mp3',
   audioQuality:    process.env.AUDIO_QUALITY   || '0',