feat(monitoring): add kube-prometheus-stack configuration and stalwart ServiceMonitor

[jzunigax2]

Adds the base observability setup for the mail cluster under deploy/monitoring/:

• kube-prometheus-stack-values.yaml: values for deploying kube-prometheus-stack (Prometheus + Grafana + Alertmanager) into a monitoring namespace. Tuned for our OVH managed cluster: control-plane scrape targets disabled (not reachable on managed k8s), 30d retention on a 20Gi PVC, and cross-namespace ServiceMonitor/rule discovery enabled.
• stalwart-servicemonitor.yaml: scrapes Stalwart's Prometheus exporter (/metrics/prometheus on the management port) every 30s through the existing stalwart ClusterIP service, authenticated via Basic auth.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
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0.0% Coverage on New Code
0.0% Duplication on New Code

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