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/local.go

@@ -0,0 +1,98 @@
+package captcha
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/mojocn/base64Captcha"
+	"github.com/redis/go-redis/v9"
+)
+
+type localService struct {
+	redis  *redis.Client
+	driver base64Captcha.Driver
+}
+
+func newLocalService(redisClient *redis.Client) Service {
+	// Configure captcha driver
+	driver := base64Captcha.NewDriverDigit(80, 240, 5, 0.7, 80)
+	return &localService{
+		redis:  redisClient,
+		driver: driver,
+	}
+}
+
+func (s *localService) Generate(ctx context.Context) (id string, image string, err error) {
+	// Generate captcha
+	captcha := base64Captcha.NewCaptcha(s.driver, &redisStore{
+		redis: s.redis,
+		ctx:   ctx,
+	})
+
+	id, b64s, answer, err := captcha.Generate()
+	if err != nil {
+		return "", "", err
+	}
+
+	// Store answer in Redis with 5 minute expiration
+	key := fmt.Sprintf("captcha:%s", id)
+	err = s.redis.Set(ctx, key, answer, 5*time.Minute).Err()
+	if err != nil {
+		return "", "", err
+	}
+
+	return id, b64s, nil
+}
+
+func (s *localService) Verify(ctx context.Context, id string, code string, ip string) (bool, error) {
+	if id == "" || code == "" {
+		return false, nil
+	}
+
+	key := fmt.Sprintf("captcha:%s", id)
+
+	// Get answer from Redis
+	answer, err := s.redis.Get(ctx, key).Result()
+	if err != nil {
+		return false, err
+	}
+
+	// Delete captcha after verification (one-time use)
+	s.redis.Del(ctx, key)
+
+	// Verify code
+	return answer == code, nil
+}
+
+func (s *localService) GetType() CaptchaType {
+	return CaptchaTypeLocal
+}
+
+// redisStore implements base64Captcha.Store interface
+type redisStore struct {
+	redis *redis.Client
+	ctx   context.Context
+}
+
+func (r *redisStore) Set(id string, value string) error {
+	key := fmt.Sprintf("captcha:%s", id)
+	return r.redis.Set(r.ctx, key, value, 5*time.Minute).Err()
+}
+
+func (r *redisStore) Get(id string, clear bool) string {
+	key := fmt.Sprintf("captcha:%s", id)
+	val, err := r.redis.Get(r.ctx, key).Result()
+	if err != nil {
+		return ""
+	}
+	if clear {
+		r.redis.Del(r.ctx, key)
+	}
+	return val
+}
+
+func (r *redisStore) Verify(id, answer string, clear bool) bool {
+	v := r.Get(id, clear)
+	return v == answer
+}