fix: add SPOTIFY_PROXY support to bypass server IP ban

Spotify blocks API calls from server IPs (open.spotify.com/api/* times
out, api.spotify.com/v1/ returns 429). Setting SPOTIFY_PROXY routes all
Spotify calls through curl_cffi via that proxy, bypassing the IP ban.

Without proxy, SpotipyFree is still tried (works on non-banned IPs).
A clear tip is printed if SpotipyFree fails without a proxy configured.

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

docker-compose.yml

@@ -30,6 +30,7 @@ services:
       - MAX_PARALLEL=${MAX_PARALLEL:-2}
       - DOWNLOAD_BACKEND=${DOWNLOAD_BACKEND:-auto}
       - SQUID_QUALITY=${SQUID_QUALITY:-mp3}
+      - SPOTIFY_PROXY=${SPOTIFY_PROXY:-}
     command: >
       sh -c "
         apt-get update && apt-get install -y --no-install-recommends python3 python3-pip ffmpeg curl git && rm -rf /var/lib/apt/lists/* &&