Consume the two-tier Authentik RBAC (terraform-authentik): request the
hierarchical `ak_groups` scope and map `akP-grafana-admin` to the Grafana Admin
role (replacing the flat `grafana-admins`). Members of akR-global-admin inherit
akP-grafana-admin. Everyone else who can log in (gated to akP-grafana-* by the
Authentik access policy) gets Viewer.