feat(stackgres): add StackGres PostgreSQL dashboard

[francisco645]

Adds a dashboard for StackGres SGClusters, built on the metrics each cluster pod already exposes (postgres_exporter pg_* / pgbouncer_*, Patroni patroni_*) plus kubeletstats pod and volume metrics.

• Sections: HA and replication (Patroni leaders, failovers, replica lag in seconds and bytes), connections and PgBouncer pooling, load and contention, I/O and cache, WAL and archiving, vacuum and XID wraparound, Kubernetes resources of the cluster pods.
• Query Builder only. No ClickHouse SQL or PromQL. Thresholds and panel descriptions say what each signal means and when to act.
• Variables: cluster, deployment_environment, namespace, pod, datname.
• README: covers ingestion with the OpenTelemetry Collector Prometheus receiver.

It comes from a dashboard we run in production. Workspace-specific values (cluster and database names, defaults, pinned series colours) were removed, so I haven't attached screenshots of the production data.