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>

docker-compose.yml

@@ -18,7 +18,7 @@ services:
     restart: unless-stopped
     working_dir: /app
     volumes:
-      - ./backend/novasonic-server.js:/app/novasonic-server.js:ro
+      - ./backend:/app:ro                # mount entire backend dir (server + lib/)
       - ${MUSIC_PATH}:${MUSIC_PATH}      # exact path from .env, no remapping
     ports:
       - "3700:3700"                      # route this port through your reverse proxy
@@ -28,9 +28,11 @@ services:
       - AUDIO_FMT=${AUDIO_FMT:-mp3}
       - AUDIO_QUALITY=${AUDIO_QUALITY:-0}
       - MAX_PARALLEL=${MAX_PARALLEL:-2}
+      - DOWNLOAD_BACKEND=${DOWNLOAD_BACKEND:-auto}
+      - SQUID_QUALITY=${SQUID_QUALITY:-mp3}
     command: >
       sh -c "
         apk add --no-cache yt-dlp ffmpeg python3 &&
-        npm install express cors &&
-        node novasonic-server.js
+        mkdir -p /deps && cd /deps && npm install express &&
+        cd /app && NODE_PATH=/deps/node_modules node novasonic-server.js
       "