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>

apps/base/authentik/gateway.yaml

@@ -0,0 +1,57 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  labels:
+    traefik.io/instance: internal
+  annotations:
+    cert-manager.io/cluster-issuer: vault-issuer
+    cert-manager.io/common-name: identity.unkin.net
+    cert-manager.io/private-key-size: "4096"
+    external-dns.alpha.kubernetes.io/hostname: identity.unkin.net,identity.k8s.syd1.au.unkin.net
+    external-dns.alpha.kubernetes.io/target: 198.18.200.4
+  name: authentik
+  namespace: authentik
+spec:
+  gatewayClassName: traefik-internal
+  listeners:
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: identity.unkin.net
+      name: http
+      port: 80
+      protocol: HTTP
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: identity.unkin.net
+      name: https
+      port: 443
+      protocol: HTTPS
+      tls:
+        certificateRefs:
+          - group: ""
+            kind: Secret
+            name: authentik-tls
+        mode: Terminate
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: identity.k8s.syd1.au.unkin.net
+      name: http-internal
+      port: 80
+      protocol: HTTP
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: identity.k8s.syd1.au.unkin.net
+      name: https-internal
+      port: 443
+      protocol: HTTPS
+      tls:
+        certificateRefs:
+          - group: ""
+            kind: Secret
+            name: authentik-tls
+        mode: Terminate