feat(API): optional display name on users

internal/domain/user/userDomain.go

@@ -13,6 +13,7 @@ var ErrVerificationCodeExpired = errors.New("verification code expired")
 var ErrAlreadyVerified = errors.New("user already verified")
 var ErrInvalidPasswordResetCode = errors.New("invalid password reset code")
 var ErrPasswordResetCodeExpired = errors.New("password reset code expired")
+var ErrInvalidDisplayName = errors.New("invalid display name")
 
 type User struct {
 	ID                      uint32    `json:"id"`
@@ -20,6 +21,7 @@ type User struct {
 	Password                string    `json:"password"`
 	PasswordResetCode       string    `json:"password_reset_code"`
 	Email                   string    `json:"email"`
+	DisplayName             string    `json:"display_name"`
 	IsVerified              bool      `json:"is_verified"`
 	VerificationCode        string    `json:"verification_code"`
 	VerificationExpiration  time.Time `json:"verification_expiration"`