refactor: simplify system role deletion validation

Remove redundant action_type check since validation already
runs only on destroy actions. Add field to error for better
error handling.

lib/mv/authorization/role.ex

@@ -88,14 +88,11 @@ defmodule Mv.Authorization.Role do
              message: "must be one of: own_data, read_only, normal_user, admin"
 
     validate fn changeset, _context ->
-               if changeset.action_type == :destroy do
-                 if changeset.data.is_system_role do
-                   {:error,
-                    message:
-                      "Cannot delete system role. System roles are required for the application to function."}
-                 else
-                   :ok
-                 end
+               if changeset.data.is_system_role do
+                 {:error,
+                  field: :is_system_role,
+                  message:
+                    "Cannot delete system role. System roles are required for the application to function."}
                else
                  :ok
                end