chore(iam): retire Authentik

Zitadel is running and has provisioned OIDC clients for all 12 consumers
(state.json: kubernetes, paperless, forgejo, argocd, grafana, immich, minio,
vault, poimen, api-gw, queue-sqs, homarr). Authentik's remaining job was to
serve until the last consumer moved.

Remove every Authentik artifact from GitOps:
- ArgoCD Application `authentik` and its Helm values
- ArgoCD Application `iam-jobs` (provisioner RBAC + migration job)
- ArgoCD Application `oidc-bootstrap` (one-shot apiserver restart)
- authentik-db CNPG cluster definition
- authentik-secrets SOPS file and secret-generator entry
- Ingress rule, cloudflared tunnel hostname, CoreDNS rewrite
- Prometheus alerts and ServiceMonitor
- admin OIDC ClusterRoleBinding provisioning binding
- scripts/iam/provision-rbac.py (the Authentik provisioner)

The chart repo https://charts.goauthentik.io is removed from the AppProject.

Kube-apiserver OIDC and consumer configs are switched to Zitadel in the
local cluster-config (gitignored) and applied separately via talosctl.

k8s/infra/databases/namespaces.yaml

@@ -1,7 +1,7 @@
 # DB clusters are wave 2 — their namespaces must exist first (their apps that
 # would CreateNamespace run later, w3/w8). Declared here so the databases App
-# creates them. authentik/vault/temporal/paperless CreateNamespace=true then
-# no-ops. poimen namespace created by poimen-root app (wave 7).
+# creates them. vault/temporal/paperless CreateNamespace=true then no-ops.
+# poimen namespace created by poimen-root app (wave 7).
 apiVersion: v1
 kind: Namespace
 metadata: