Add mail namespace with Stalwart backing stores (#538)

Stalwart needs a Postgres metadata store and an S3 blob store before it can be deployed into the new `mail` namespace.

- add CNPG Cluster `stalwart-db` (db/owner `stalwart`, CNPG-minted `stalwart-db-app` creds) with barman backups to a dedicated `cnpg-mail` bucket and a nightly ScheduledBackup
- add cephrgw Bucket `stalwart-blobs` with an owner user writing RW keys to `stalwart-blobs-s3`
- add the au-syd1 overlay, platform ApplicationSet path and project destination

Reviewed-on: #538
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

argocd/projects/platform.yaml

@@ -49,6 +49,8 @@ spec:
       server: https://kubernetes.default.svc
     - namespace: 'kanidm'
       server: https://kubernetes.default.svc
+    - namespace: 'mail'
+      server: https://kubernetes.default.svc
     - namespace: 'netbox'
       server: https://kubernetes.default.svc
     - namespace: 'node-feature-discovery'