The loginWithMailLink endpoint returns the magic login link in the
HTTP response body, allowing unauthenticated account takeover.
The fix returns true instead of the link. The email delivery is
the authentication factor.
Bug inherited from V2Board commit 9b707aaf (2022-06-27).
- Implement reCAPTCHA v3 with score-based validation
- Add Cloudflare Turnstile as captcha alternative
- Create reusable CaptchaService for unified validation
- Support switching between recaptcha, recaptcha-v3, and turnstile
- Maintain backward compatibility with existing configurations
