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/httproute.yaml

@@ -0,0 +1,16 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: stalwart
+  namespace: stalwart
+spec:
+  parentRefs:
+    - name: stalwart
+      namespace: stalwart
+  hostnames:
+    - mail.k8s.syd1.au.unkin.net
+  rules:
+    - backendRefs:
+        - name: stalwart
+          port: 8080