feat(security): implement Cloudflare Turnstile CAPTCHA on contact form

- Add Turnstile script to frontend/index.html
- Implement programmatic widget rendering in ContactPage
- Add backend verification via Cloudflare API
- Include client IP in verification request
- Update CSP headers to allow Cloudflare resources
- Add environment variable configuration for site and secret keys
- Pass VITE_TURNSTILE_SITE_KEY to frontend container
- Add validation and error handling for CAPTCHA tokens

docker-compose.yml

@@ -64,6 +64,7 @@ services:
       - NODE_ENV=development
       - VITE_HOST=0.0.0.0
       - VITE_ALLOWED_HOSTS=${VITE_ALLOWED_HOSTS:-all}
+      - VITE_TURNSTILE_SITE_KEY=${VITE_TURNSTILE_SITE_KEY}
     stdin_open: true
     tty: true
     command: npm run dev