feat: add email verify code endpoint and require code on register

- POST /v1/user/email/code sends a 6-digit verify code via email (rate-limited, 10min TTL)
- RegisterByEmail now validates verify_code before creating the account
- Added email code cache helpers mirroring SMS pattern
- Added error codes 1007 (email code error) and 1008 (send limit)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/storage/cache/user.go

@@ -61,3 +61,55 @@ func SetUserSendSmsLimit(ctx context.Context, phone string) error {
 func DeleteUserSmsCode(ctx context.Context, phone string) error {
 	return RedisClient.Del(ctx, fmt.Sprintf(UserSmsCodePrefix, phone)).Err()
 }
+
+const (
+	UserEmailCodeTTL         = 10 * time.Minute
+	UserSendEmailLimitTTL    = 24 * time.Hour
+	UserSendEmailLimitCount  = 5
+)
+
+const (
+	UserEmailCodePrefix = "user:email_code:%s"
+	UserSendEmailLimit  = "user:send_email_limit:%s"
+)
+
+func GetUserEmailCode(ctx context.Context, email string) (string, error) {
+	code, err := RedisClient.Get(ctx, fmt.Sprintf(UserEmailCodePrefix, email)).Result()
+	if err != nil {
+		if err == redis.Nil {
+			return "", nil
+		}
+		return "", err
+	}
+	return code, nil
+}
+
+func SetUserEmailCode(ctx context.Context, email, code string) error {
+	return RedisClient.Set(ctx, fmt.Sprintf(UserEmailCodePrefix, email), code, UserEmailCodeTTL).Err()
+}
+
+func GetUserSendEmailLimit(ctx context.Context, email string) (int, error) {
+	limit, err := RedisClient.Get(ctx, fmt.Sprintf(UserSendEmailLimit, email)).Result()
+	if err != nil {
+		if err == redis.Nil {
+			return 0, nil
+		}
+		return 0, err
+	}
+	return strconv.Atoi(limit)
+}
+
+func SetUserSendEmailLimit(ctx context.Context, email string) error {
+	count, err := RedisClient.Incr(ctx, fmt.Sprintf(UserSendEmailLimit, email)).Result()
+	if err != nil {
+		return err
+	}
+	if count > UserSendEmailLimitCount {
+		return errors.New("send email limit")
+	}
+	return RedisClient.Expire(ctx, fmt.Sprintf(UserSendEmailLimit, email), UserSendEmailLimitTTL).Err()
+}
+
+func DeleteUserEmailCode(ctx context.Context, email string) error {
+	return RedisClient.Del(ctx, fmt.Sprintf(UserEmailCodePrefix, email)).Err()
+}