ReCAPTCHA V3与V2混合部署策略：为低分用户提供挑战机制

<!-- ReCAPTCHA V3 脚本 -->
<script src="https://www.google.com/recaptcha/api.js?render=YOUR_V3_SITE_KEY"></script>

<!-- ReCAPTCHA V2 脚本 (在需要时加载，或预加载但隐藏) -->
<script src="https://www.google.com/recaptcha/api.js?onload=onloadCallback&render=explicit" async defer></script>

<form id="myForm" action="/submit" method="POST">
    <!-- V3 token 占位符，由JS填充 -->
    <input type="hidden" name="recaptcha_v3_token" id="recaptchaV3Token">

    <!-- V2 挑战容器，初始隐藏 -->
    <div id="recaptchaV2Container" style="display: none;">
        <div class="g-recaptcha" data-sitekey="YOUR_V2_SITE_KEY" data-callback="onV2Success"></div>
    </div>

    <button type="submit">提交</button>
</form>

// V2 回调函数 (当V2挑战成功时触发)
function onV2Success(token) {
    // 将V2 token发送到服务器进行验证
    document.getElementById('myForm').submit(); // 或通过AJAX发送
}

// ReCAPTCHA V3 执行并发送到服务器
document.getElementById('myForm').addEventListener('submit', function(event) {
    event.preventDefault(); // 阻止表单默认提交

    grecaptcha.ready(function() {
        grecaptcha.execute('YOUR_V3_SITE_KEY', {action: 'submit_form'}).then(function(token) {
            document.getElementById('recaptchaV3Token').value = token;

            // 将V3 token发送到服务器进行验证
            fetch('/verify-recaptcha', {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json',
                },
                body: JSON.stringify({ recaptchaV3Token: token })
            })
            .then(response => response.json())
            .then(data => {
                if (data.score < 0.5 && data.success) { // 假设阈值为0.5，且V3验证本身成功
                    // V3分数过低，显示V2挑战
                    document.getElementById('recaptchaV2Container').style.display = 'block';
                    // 如果V2是显式渲染，需要调用 grecaptcha.render
                    // grecaptcha.render('recaptchaV2Container', {
                    //     'sitekey' : 'YOUR_V2_SITE_KEY',
                    //     'callback'