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/redis-deployment.yaml

@@ -0,0 +1,58 @@
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: redis
+  namespace: authentik
+spec:
+  replicas: 1
+  strategy:
+    type: Recreate
+  selector:
+    matchLabels:
+      app: redis
+  template:
+    metadata:
+      labels:
+        app: redis
+    spec:
+      containers:
+        - name: redis
+          image: redis:7-alpine
+          imagePullPolicy: IfNotPresent
+          args:
+            - --save
+            - "20"
+            - "1"
+          ports:
+            - containerPort: 6379
+              name: redis
+              protocol: TCP
+          livenessProbe:
+            exec:
+              command:
+                - redis-cli
+                - ping
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          readinessProbe:
+            exec:
+              command:
+                - redis-cli
+                - ping
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          resources:
+            limits:
+              cpu: 500m
+              memory: 512Mi
+            requests:
+              cpu: 50m
+              memory: 128Mi
+          volumeMounts:
+            - mountPath: /data
+              name: redis-data
+      volumes:
+        - name: redis-data
+          persistentVolumeClaim:
+            claimName: redis-data