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.

terraform/files/coredns/Corefile

@@ -22,7 +22,6 @@
     # TLS termination (cert-manager cert) and preserve the Host header. Routing
     # via nginx (not the backend service) is critical: a direct rewrite bypasses
     # nginx TLS and exposes each app's own self-signed cert, trusted by nothing.
-    rewrite name authentik.riotpiao.com ingress-nginx-controller.ingress-nginx.svc.cluster.local
     rewrite name zitadel.riotpiao.com ingress-nginx-controller.ingress-nginx.svc.cluster.local
     rewrite name grafana.riotpiao.com ingress-nginx-controller.ingress-nginx.svc.cluster.local
     rewrite name minio.riotpiao.com ingress-nginx-controller.ingress-nginx.svc.cluster.local