Add Authentik identity provider deployment

- Helm chart authentik 2026.5.3 with 3 server replicas, 2 worker replicas
- CNPG PostgreSQL cluster (3 instances) with rw and ro poolers (2 instances each)
- Redis with 5Gi persistent storage
- Gateway API: identity.unkin.net and identity.k8s.syd1.au.unkin.net (HTTPS)
- LDAPS via TLSRoute on ldap.k8s.syd1.au.unkin.net and ldap.main.unkin.net
- Multi-SAN TLS via cert-manager gateway integration
- S3 storage via RadosGW (bucket: authentik)
- Vault secrets: postgres-credentials, authentik-credentials, s3-credentials
- Woodpecker ServiceAccount for terraform-authentik CI
- Platform applicationset and project updated

argocd/applicationsets/platform.yaml

@@ -10,6 +10,7 @@ spec:
       repoURL: https://git.unkin.net/unkin/argocd-apps
       revision: HEAD
       directories:
+      - path: apps/overlays/*/authentik
       - path: apps/overlays/*/artifactapi
       - path: apps/overlays/*/age-api
       - path: apps/overlays/*/cattle-system