Add Authentik identity provider deployment (#211)

## Summary
- Deploy Authentik (identity.unkin.net) via Helm chart 2026.5.3
- CNPG PostgreSQL cluster (3 instances) with separate rw/ro poolers (2 instances each)
- Redis with 5Gi persistent storage
- Gateway API for HTTPS (identity.unkin.net) and LDAPS (ldap.k8s.syd1.au.unkin.net, ldap.main.unkin.net)
- TLSRoute for LDAPS passthrough, HTTPRoute for external-dns record creation
- Vault secrets for postgres credentials, authentik secret key, and S3 storage credentials
- S3 storage via RadosGW (bucket: authentik)
- 3 server replicas, 2 worker replicas
- Woodpecker ServiceAccount for terraform-authentik CI
- Platform applicationset and project updated

## Dependencies
- terraform-git #15 (merged) — repo definition
- terraform-vault #78 (merged) — auth roles and Consul ACL

## Vault secrets needed before deploy
Write to `kv/kubernetes/namespace/authentik/default/`:
- `postgres-credentials`: username + password
- `authentik-credentials`: AUTHENTIK_SECRET_KEY
- `s3-credentials`: S3 access key + secret key

Reviewed-on: #211
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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