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)

argocd/applicationsets/platform.yaml

@@ -26,6 +26,7 @@ spec:
       - path: apps/overlays/*/reflector-system
       - path: apps/overlays/*/reloader-system
       - path: apps/overlays/*/reposync
+      - path: apps/overlays/*/stalwart
       - path: apps/overlays/*/traefik-system
       - path: apps/overlays/*/vm-system
       - path: apps/overlays/*/vault