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/certificate.yaml

@@ -0,0 +1,36 @@
+---
+# TLS cert for stalwart's own HTTPS/IMAPS/STARTTLS listeners (mail.main.unkin.net)
+apiVersion: cert-manager.io/v1
+kind: Certificate
+metadata:
+  name: stalwart-tls
+  namespace: stalwart
+spec:
+  secretName: stalwart-tls
+  issuerRef:
+    name: vault-issuer
+    kind: ClusterIssuer
+  commonName: mail.main.unkin.net
+  dnsNames:
+    - mail.main.unkin.net
+  privateKey:
+    size: 4096
+    algorithm: RSA
+---
+# TLS cert for Traefik Gateway (internal k8s admin URL)
+apiVersion: cert-manager.io/v1
+kind: Certificate
+metadata:
+  name: stalwart-gateway-tls
+  namespace: stalwart
+spec:
+  secretName: stalwart-gateway-tls
+  issuerRef:
+    name: vault-issuer
+    kind: ClusterIssuer
+  commonName: mail.k8s.syd1.au.unkin.net
+  dnsNames:
+    - mail.k8s.syd1.au.unkin.net
+  privateKey:
+    size: 4096
+    algorithm: RSA