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/service.go

@@ -0,0 +1,49 @@
+package captcha
+
+import (
+	"context"
+
+	"github.com/redis/go-redis/v9"
+)
+
+type CaptchaType string
+
+const (
+	CaptchaTypeLocal     CaptchaType = "local"
+	CaptchaTypeTurnstile CaptchaType = "turnstile"
+)
+
+// Service defines the captcha service interface
+type Service interface {
+	// Generate generates a new captcha
+	// For local captcha: returns id and base64 image
+	// For turnstile: returns empty strings
+	Generate(ctx context.Context) (id string, image string, err error)
+
+	// Verify verifies the captcha
+	// For local captcha: token is captcha id, code is user input
+	// For turnstile: token is cf-turnstile-response, code is ignored
+	Verify(ctx context.Context, token string, code string, ip string) (bool, error)
+
+	// GetType returns the captcha type
+	GetType() CaptchaType
+}
+
+// Config holds the configuration for captcha service
+type Config struct {
+	Type         CaptchaType
+	RedisClient  *redis.Client
+	TurnstileSecret string
+}
+
+// NewService creates a new captcha service based on the config
+func NewService(config Config) Service {
+	switch config.Type {
+	case CaptchaTypeTurnstile:
+		return newTurnstileService(config.TurnstileSecret)
+	case CaptchaTypeLocal:
+		fallthrough
+	default:
+		return newLocalService(config.RedisClient)
+	}
+}