feat: add email verification code for registration and optimize email service

- Add POST /user/email/code endpoint to send 6-digit verification code
- Require email code verification before completing registration
- Add email code cache with 10min TTL and 5/day send rate limit
- Fix nil client guard, TLS conn leak, domain parsing, and Resend error body in email pkg
- Deploy via ssh inline command using current branch

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

internal/storage/cache/user.go

@@ -61,3 +61,56 @@ 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 DeleteUserEmailCode(ctx context.Context, email string) error {
+	return RedisClient.Del(ctx, fmt.Sprintf(UserEmailCodePrefix, email)).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 {
+	key := fmt.Sprintf(UserSendEmailLimit, email)
+	count, err := RedisClient.Incr(ctx, key).Result()
+	if err != nil {
+		return err
+	}
+	if count > UserSendEmailLimitCount {
+		return errors.New("send email limit")
+	}
+	return RedisClient.Expire(ctx, key, UserSendEmailLimitTTL).Err()
+}