Pixiportal

As an administrator, I want to assign roles to users for each application, so that users receive the appropriate access rights.

Acceptance Criteria:
1: Assign a role to a user
Given a user exists, an application exists, and a role exists for this application,
When the administrator assigns the role to the user
Then the system associates the user with the role for the selected application
And the user inherits the role's predefined permissions.

2: Assign multiple roles to a user:
Given a user already has a role assigned in an application
When the administrator assigns another role to the same user
Then the system associates both roles with the user
And the user receives the combined permissions from all assigned roles.

3: Remove a role from a user:
Given a user has an assigned role
When the administrator removes the role assignment
Then the system removes the association between the user and the role
And the user loses the permissions provided by this role.