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.
A UK buyer can now create their own account at /portal/signup/. The screen
creates a Keycloak user through a least-privilege service account
(ixg-provisioning: manage-users, view-users) and the matching Django row as
a buyer, then sends them through the normal sign-in flow to the portal.
Staff status is derived from the role in the OIDC backend rather than
asserted, so a self-registered buyer is not staff and does not become staff
on a later sign-in. The platform root and the RBAC middleware now route
each role to its own portal, which also fixes a redirect loop that the
staff fix exposed for non-staff buyers.
