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/services/api.js

@@ -112,10 +112,10 @@ async function fetchAPI(endpoint, options = {}) {
 
 // Auth API
 export const authAPI = {
-  async register(username, email, password, firstName = null, lastName = null, wsdcId = null) {
+  async register(username, email, password, firstName = null, lastName = null, wsdcId = null, turnstileToken = null) {
     const data = await fetchAPI('/auth/register', {
       method: 'POST',
-      body: JSON.stringify({ username, email, password, firstName, lastName, wsdcId }),
+      body: JSON.stringify({ username, email, password, firstName, lastName, wsdcId, turnstileToken }),
     });
 
     // Save token