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

nginx/conf.d/default.conf

@@ -20,7 +20,7 @@ server {
     add_header Permissions-Policy "geolocation=(), microphone=(), camera=()" always;
 
     # Content Security Policy (permissive for dev, tighten for production)
-    add_header Content-Security-Policy "default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self' data:; connect-src 'self' ws: wss:; media-src 'self' blob:; object-src 'none'; base-uri 'self'; form-action 'self';" always;
+    add_header Content-Security-Policy "default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval' https://challenges.cloudflare.com; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self' data:; connect-src 'self' ws: wss: https://challenges.cloudflare.com; media-src 'self' blob:; object-src 'none'; base-uri 'self'; form-action 'self'; frame-src https://challenges.cloudflare.com;" always;
 
     # Block access to hidden files and directories (but allow .vite for development)
     location ~ /\.(git|svn|htaccess|htpasswd|env) {