Both permission sets allow User:update :own, so users should be able
to access their profile page. This makes the implementation consistent
with the documentation and the logical permission model.
- Add types for scope, action, resource_permission, permission_set
- Implement get_permissions/1 for all 4 sets (own_data, read_only, normal_user, admin)
- Implement valid_permission_set?/1 for string and atom validation
- Implement permission_set_name_to_atom/1 with error handling