feat: switch Spotify auth to official Client Credentials Flow

Replaces the broken open.spotify.com/get_access_token scrape with
accounts.spotify.com/api/token (Basic auth, client_credentials grant).
Adds SPOTIFY_CLIENT_ID/SECRET to config, docker-compose, and .env.example.
Health endpoint now reports spotify.configured; frontend disables the
importer and shows a setup guide when credentials are missing.

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

docker-compose.yml

@@ -30,6 +30,8 @@ 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 &&