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>

.env.example

@@ -22,3 +22,10 @@ DOWNLOAD_BACKEND=auto
 #   flac   - FLAC 16-bit / 44.1 kHz
 #   hi-res - FLAC 24-bit / 96 kHz
 SQUID_QUALITY=mp3
+
+# Spotify Importer (optional — free, no Premium needed)
+# 1. Go to https://developer.spotify.com/dashboard and create an app
+# 2. Set any name, Redirect URI: http://localhost
+# 3. Copy the Client ID and Client Secret below
+SPOTIFY_CLIENT_ID=
+SPOTIFY_CLIENT_SECRET=