헬스체크 추가

backend/internal/service/redis_service.go

@@ -33,6 +33,13 @@ func NewRedisService() (*RedisService, error) {
 	return &RedisService{Client: rdb}, nil
 }
 
+func (s *RedisService) Ping(ctx context.Context) error {
+	if s.Client == nil {
+		return os.ErrInvalid
+	}
+	return s.Client.Ping(ctx).Err()
+}
+
 // StoreVerificationCode saves the SMS verification code with a 3-minute expiration
 func (s *RedisService) StoreVerificationCode(phone, code string) error {
 	// Key format: "sms_verify:01012345678"