fix: switch backend to node:20-slim (Debian) for glibc compatibility

tls-client (used by SpotAPI/spotdl) bundles a native .so that requires
libresolv.so.2 from glibc. Alpine uses musl libc which doesn't provide
this — causing spotdl to crash on import. Debian-based image has glibc.
Also install yt-dlp via direct download (latest) instead of apk.

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

docker-compose.yml

@@ -13,7 +13,7 @@ services:
 
   # ── Backend (yt-dlp download server) ─────────────────────────────────────────
   novasonic-backend:
-    image: node:20-alpine
+    image: node:20-slim
     container_name: novasonic-backend
     restart: unless-stopped
     working_dir: /app
@@ -30,11 +30,10 @@ services:
       - MAX_PARALLEL=${MAX_PARALLEL:-2}
       - DOWNLOAD_BACKEND=${DOWNLOAD_BACKEND:-auto}
       - SQUID_QUALITY=${SQUID_QUALITY:-mp3}
-      - SPOTIFY_CLIENT_ID=${SPOTIFY_CLIENT_ID:-}
-      - SPOTIFY_CLIENT_SECRET=${SPOTIFY_CLIENT_SECRET:-}
     command: >
       sh -c "
-        apk add --no-cache yt-dlp ffmpeg python3 py3-pip py3-requests git gcc musl-dev python3-dev libffi-dev &&
+        apt-get update && apt-get install -y --no-install-recommends python3 python3-pip ffmpeg curl git && rm -rf /var/lib/apt/lists/* &&
+        curl -sL https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp -o /usr/local/bin/yt-dlp && chmod a+rx /usr/local/bin/yt-dlp &&
         pip install --break-system-packages 'git+https://github.com/TzurSoffer/spotify-downloader' &&
         mkdir -p /deps && cd /deps && npm install express &&
         cd /app && NODE_PATH=/deps/node_modules node novasonic-server.js