feat(API): optional display name on users

internal/infrastructure/persistence/models/userModel.go

@@ -10,6 +10,7 @@ type User struct {
 	Password                string  `gorm:"not null"`
 	PasswordResetCode       *string `gorm:"unique"`
 	Email                   string  `gorm:"unique;not null"`
+	DisplayName             string  `gorm:"not null;default:''"`
 	IsVerified              bool    `gorm:"default=false;not null"`
 	VerificationCode        *string `gorm:"unique"`
 	VerificationExpiration  time.Time