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>