feat(stalwart): deploy Stalwart mail server with CNPG and S3

- stalwart namespace with Deployment + HPA (2-6 replicas)
- CNPG PostgreSQL cluster (3 instances, 20Gi cephrbd-fast-delete) with PgBouncer pooler
- S3/Ceph-RGW for blob storage (stalwart-maildata bucket, lz4 compressed)
- Secrets from Vault: postgres-credentials, s3-credentials, stalwart-admin
- TLS cert via cert-manager (vault-issuer) for mail.main.unkin.net
- SMTP relay on port 25 (internal ClusterIP, trusted pod CIDRs)
- Submission on port 587, IMAP 143/993, HTTPS 443 via LoadBalancer
- HTTP port 8080 for Traefik reverse proxy (web admin at mail.k8s.syd1.au.unkin.net)
- Outbound mail routed through postfix.mailgateway.svc.cluster.local:25
- Spam filtering offloaded to postfix/rspamd (disabled internally)

apps/base/stalwart/stalwart-hpa.yaml

@@ -0,0 +1,38 @@
+---
+apiVersion: autoscaling/v2
+kind: HorizontalPodAutoscaler
+metadata:
+  name: stalwart-hpa
+  namespace: stalwart
+spec:
+  scaleTargetRef:
+    apiVersion: apps/v1
+    kind: Deployment
+    name: stalwart
+  minReplicas: 2
+  maxReplicas: 6
+  metrics:
+    - type: Resource
+      resource:
+        name: cpu
+        target:
+          type: Utilization
+          averageUtilization: 70
+  behavior:
+    scaleUp:
+      stabilizationWindowSeconds: 0
+      selectPolicy: Max
+      policies:
+        - type: Percent
+          value: 100
+          periodSeconds: 60
+        - type: Pods
+          value: 2
+          periodSeconds: 30
+    scaleDown:
+      stabilizationWindowSeconds: 300
+      selectPolicy: Min
+      policies:
+        - type: Percent
+          value: 30
+          periodSeconds: 60