fix: allow deletion of inactive admin users (ENT-11714)

- Removed auth_user.is_active=True filter from admin listing query
  so inactive admin users appear in the admin list
- Added EnterpriseCustomerUser.active=True and enterprise_admin role
  check via SystemWideEnterpriseUserRoleAssignment
- Changed hard delete to log warning instead of returning 404 when
  admin role assignment is missing
- Updated tests accordingly