security: remove is_system_role from public API

Remove is_system_role from accept lists in create_role and update_role
actions. This field should only be set via seeds or internal actions to
prevent users from creating unkillable roles through the public API.

lib/accounts/user.ex

@@ -17,6 +17,10 @@ defmodule Mv.Accounts.User do
       # When a member is deleted, set the user's member_id to NULL
       # This allows users to continue existing even if their linked member is removed
       reference :member, on_delete: :nilify
+
+      # When a role is deleted, prevent deletion if users are assigned to it
+      # This protects critical roles from accidental deletion
+      reference :role, on_delete: :restrict
     end
   end