* Add Cap Captcha support
- Add CaptchaCap type constant in types.go
- Add Cap struct with InstanceURL, KeyID, and KeySecret fields
- Add CapCaptcha method in provider.go to return Cap settings
- Add default settings for Cap captcha in setting.go
- Implement Cap captcha verification logic in middleware
- Expose Cap captcha settings in site API
This adds support for Cap captcha service as an alternative
captcha option alongside existing reCAPTCHA, Turnstile and
built-in captcha options.
* update cap json tags
