feat(security): add Cloudflare Turnstile CAPTCHA to registration form

- Add Turnstile widget rendering in RegisterPage on step 2
- Implement programmatic widget initialization with callbacks
- Add token validation before form submission
- Update AuthContext and API service to pass turnstileToken
- Add backend verification via Cloudflare API in register controller
- Include client IP in verification request
- Add validation rule for turnstileToken
- Reset widget on registration error

frontend/src/contexts/AuthContext.jsx

@@ -48,9 +48,9 @@ export const AuthProvider = ({ children }) => {
     }
   };
 
-  const register = async (username, email, password, firstName = null, lastName = null, wsdcId = null) => {
+  const register = async (username, email, password, firstName = null, lastName = null, wsdcId = null, turnstileToken = null) => {
     try {
-      const { user: userData } = await authAPI.register(username, email, password, firstName, lastName, wsdcId);
+      const { user: userData } = await authAPI.register(username, email, password, firstName, lastName, wsdcId, turnstileToken);
       setUser(userData);
       // Save to localStorage for persistence
       localStorage.setItem('user', JSON.stringify(userData));