feat: split backend into lib/ modules + add squid.wtf (Qobuz) download backend

- backend/lib/config.js  — CONFIG from env (adds DOWNLOAD_BACKEND, SQUID_QUALITY)
- backend/lib/jobs.js    — jobs Map, updateJob(), log()
- backend/lib/metadata.js — iTunes, artwork, lyrics, sanitize helpers
- backend/lib/ytdlp.js   — yt-dlp download cascade + shared embedMetadata()
- backend/lib/squid.js   — ALTCHA PoW solver, Qobuz search/download via squid.wtf
- backend/lib/downloader.js — backend selector: ytdlp | squid | auto (squid-first + fallback)
- novasonic-server.js thinned to Express routes only, imports from lib/
- docker-compose: mount full backend/ dir, install deps outside read-only volume
- .env.example: document DOWNLOAD_BACKEND and SQUID_QUALITY

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

backend/lib/config.js

@@ -0,0 +1,14 @@
+const CONFIG = {
+  port:            process.env.PORT             || 3700,
+  musicPath:       process.env.MUSIC_PATH       || '/music',
+  ytdlpBin:        process.env.YTDLP_BIN       || 'yt-dlp',
+  audioFmt:        process.env.AUDIO_FMT       || 'mp3',
+  audioQuality:    process.env.AUDIO_QUALITY   || '0',
+  maxParallel:     parseInt(process.env.MAX_PARALLEL || '2', 10),
+  // 'ytdlp' | 'squid' | 'auto' (squid first, yt-dlp fallback)
+  downloadBackend: (process.env.DOWNLOAD_BACKEND || 'auto').toLowerCase(),
+  // squid.wtf quality: 'mp3' | 'flac' | 'hi-res'
+  squidQuality:    (process.env.SQUID_QUALITY   || 'mp3').toLowerCase(),
+};
+
+module.exports = CONFIG;