Add nginx HTTP Basic Auth for production

A custom entrypoint generates /etc/nginx/.htpasswd at container startup
from AUTH_USER and AUTH_PASSWORD env vars (via openssl). No credentials
are baked into the image. Pass AUTH_USER and AUTH_PASSWORD in docker-compose.

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

.env.example

@@ -3,3 +3,7 @@ FOOTBALL_API_KEY=your_api_key_here
 
 # Only needed for local development (npm run dev)
 VITE_FOOTBALL_API_KEY=your_api_key_here
+
+# Basic auth credentials (production only)
+AUTH_USER=henry
+AUTH_PASSWORD=your_password_here