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.
