Add Cap Captcha support (#2511)

* 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

pkg/setting/types.go

@@ -28,6 +28,7 @@ const (
 	CaptchaReCaptcha = CaptchaType("recaptcha")
 	CaptchaTcaptcha  = CaptchaType("tcaptcha")
 	CaptchaTurnstile = CaptchaType("turnstile")
+	CaptchaCap       = CaptchaType("cap")
 )
 
 type ReCaptcha struct {
@@ -47,6 +48,12 @@ type Turnstile struct {
 	Secret string
 }
 
+type Cap struct {
+	InstanceURL string
+	KeyID       string
+	KeySecret   string
+}
+
 type SMTP struct {
 	FromName        string
 	From            string