RBACMiddleware only guarded non-staff users, so any staff account stepped
past the role routing for /admin/ — an ops or buyer account with the flag
set could reach the admin index, which embeds the model registry, and a
later permission grant would have been inherited. Restrict /admin/ to the
admin role (superuser excepted) and hide the ops Admin link from non-admins.
Also close the second finding: the seed set is_staff only on creation, so
a buyer whose flag had drifted stayed staff on every re-run. Derive it from
the role on the update path too, and align existing rows with a one-off
migration so the flag never disagrees with the role.