refactor(app): Potential race conditions

[MR2011]

Task Description

We use the following pattern across the CRUD operations in the app layer:

...
       users, err := u.ListUsers(f, &entity.ListOptions{})

	if err != nil {
		l.Error(err)
		return nil, NewUserHandlerError("Internal error while creating user.")
	}

	if len(users.Elements) > 0 {
		return nil, NewUserHandlerError(fmt.Sprintf("Duplicated entry %s for UniqueUserID.", user.UniqueUserID))
	}

	newUser, err := u.database.CreateUser(user)
...

Between the check if the user exists and the creation might occur a race condition. However, since we have a unique constraint on the database level, this will be prevented. In the future, we should avoid patterns these patterns by using transactions on the database layer.