feat: add homelab-wide Authentik RBAC model and k8s OIDC auth wiring

Adds permissions claim + per-service admin groups in Authentik, scoped
Role/RoleBinding per service, public PKCE kubernetes OAuth2 client, and
kube-apiserver OIDC extraArgs. Also fixes paperless OIDC signup permissions
via adapter override and adds CoreDNS rewrite for authentik.riotpiao.com.

k8s/infra/rbac/portainer-operator-role.yaml

@@ -0,0 +1,27 @@
+# Scoped operator access for portainer-admins: just the Deployment - no
+# configmap/secret exists for portainer today. Inert until kube-apiserver's
+# OIDC wiring lands.
+apiVersion: rbac.authorization.k8s.io/v1
+kind: Role
+metadata:
+  name: portainer-operator
+  namespace: dashboard
+rules:
+  - apiGroups: ["apps"]
+    resources: ["deployments"]
+    resourceNames: ["portainer"]
+    verbs: ["get", "list", "watch", "update", "patch"]
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: portainer-admins-binding
+  namespace: dashboard
+subjects:
+  - kind: Group
+    name: "oidc:portainer-admins"
+    apiGroup: rbac.authorization.k8s.io
+roleRef:
+  kind: Role
+  name: portainer-operator
+  apiGroup: rbac.authorization.k8s.io