feat: grant staff access to superusers automatically (#38848)

* feat: grant staff access to superusers automatically

Django's is_superuser and is_staff are independent flags, so a superuser
does not get the is_staff access that gates the Django admin and various
staff-only Studio/LMS views. Since a superuser can grant itself is_staff
at any time, that split offers no protection and is only surprising.

Add a pre_save signal on the User model that marks any superuser as staff,
and a data migration to backfill existing superusers.

Discussion: https://discuss.openedx.org/t/shouldnt-superuser-automatically-inherit-staff-access-in-open-edx/18657

* refactor: address review feedback on superuser staff grant

- Reword the receiver, migration, and test docstrings to describe the
  behaviour as grant-only rather than keeping is_staff "in sync" with
  is_superuser, since demoting a superuser never revokes is_staff.
- Exercise the pre_save receiver with an explicit save() in the
  non-superuser test instead of relying only on factory creation.
- Add test_demotion_does_not_remove_staff to lock in the one-directional
  contract.