feat(captcha): add captcha service interface and implementations

- Add captcha service interface with Generate and Verify methods
- Implement local image captcha using base64Captcha library
- Implement Cloudflare Turnstile verification wrapper
- Support Redis-based captcha storage with 5-minute expiration
- Add factory method for creating captcha service instances

pkg/captcha/turnstile.go

@@ -0,0 +1,37 @@
+package captcha
+
+import (
+	"context"
+
+	"github.com/perfect-panel/server/pkg/turnstile"
+)
+
+type turnstileService struct {
+	service turnstile.Service
+}
+
+func newTurnstileService(secret string) Service {
+	return &turnstileService{
+		service: turnstile.New(turnstile.Config{
+			Secret: secret,
+		}),
+	}
+}
+
+func (s *turnstileService) Generate(ctx context.Context) (id string, image string, err error) {
+	// Turnstile doesn't need server-side generation
+	return "", "", nil
+}
+
+func (s *turnstileService) Verify(ctx context.Context, token string, code string, ip string) (bool, error) {
+	if token == "" {
+		return false, nil
+	}
+
+	// Verify with Cloudflare Turnstile
+	return s.service.Verify(ctx, token, ip)
+}
+
+func (s *turnstileService) GetType() CaptchaType {
+	return CaptchaTypeTurnstile
+}