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/pages/RegisterPage.jsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useRef } from 'react';
 import { useNavigate, Link } from 'react-router-dom';
 import { useAuth } from '../contexts/AuthContext';
 import { wsdcAPI } from '../services/api';
@@ -32,6 +32,8 @@ const RegisterPage = () => {
   });
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState('');
+  const [turnstileToken, setTurnstileToken] = useState('');
+  const turnstileRef = useRef(null);
 
   const { register } = useAuth();
   const navigate = useNavigate();
@@ -78,6 +80,58 @@ const RegisterPage = () => {
     return () => clearTimeout(timeoutId);
   }, [wsdcId]);
 
+  // Setup Turnstile callbacks and render widget when on step 2
+  useEffect(() => {
+    if (step !== 2) return;
+
+    // Callback when Turnstile verification is successful
+    window.onTurnstileSuccess = (token) => {
+      setTurnstileToken(token);
+    };
+
+    // Callback when Turnstile encounters an error
+    window.onTurnstileError = () => {
+      setError('CAPTCHA verification failed. Please try again.');
+      setTurnstileToken('');
+    };
+
+    // Wait for Turnstile script to load and render widget
+    const renderTurnstile = () => {
+      if (window.turnstile && turnstileRef.current && !turnstileRef.current.hasChildNodes()) {
+        window.turnstile.render(turnstileRef.current, {
+          sitekey: import.meta.env.VITE_TURNSTILE_SITE_KEY,
+          callback: window.onTurnstileSuccess,
+          'error-callback': window.onTurnstileError,
+          theme: 'light',
+        });
+      }
+    };
+
+    // Check if script is already loaded
+    if (window.turnstile) {
+      renderTurnstile();
+    } else {
+      // Wait for script to load
+      const checkTurnstile = setInterval(() => {
+        if (window.turnstile) {
+          renderTurnstile();
+          clearInterval(checkTurnstile);
+        }
+      }, 100);
+
+      return () => {
+        clearInterval(checkTurnstile);
+        delete window.onTurnstileSuccess;
+        delete window.onTurnstileError;
+      };
+    }
+
+    return () => {
+      delete window.onTurnstileSuccess;
+      delete window.onTurnstileError;
+    };
+  }, [step]);
+
   // Handle WSDC ID confirmation and continue to registration
   const handleWsdcContinue = () => {
     if (!wsdcPreview) {
@@ -130,6 +184,12 @@ const RegisterPage = () => {
       return;
     }
 
+    // Validate Turnstile token
+    if (!turnstileToken) {
+      setError('Please complete the CAPTCHA verification');
+      return;
+    }
+
     setLoading(true);
 
     try {
@@ -139,11 +199,17 @@ const RegisterPage = () => {
         formData.password,
         formData.firstName || null,
         formData.lastName || null,
-        wsdcData?.wsdcId || null
+        wsdcData?.wsdcId || null,
+        turnstileToken
       );
       navigate('/events');
     } catch (err) {
       setError(err.message || 'Registration failed');
+      // Reset Turnstile on error
+      if (window.turnstile && turnstileRef.current) {
+        window.turnstile.reset(turnstileRef.current);
+        setTurnstileToken('');
+      }
     } finally {
       setLoading(false);
     }
@@ -376,6 +442,11 @@ const RegisterPage = () => {
             required
           />
 
+          {/* Cloudflare Turnstile CAPTCHA */}
+          <div>
+            <div ref={turnstileRef}></div>
+          </div>
+
           <div className="space-y-3 pt-2">
             <LoadingButton
               type="submit"