feat(demo): Prometheus + Grafana with a provisioned dashboard

make demo-ui now also starts Prometheus (scrapes coordinator:8080/metrics) and
Grafana with a provisioned datasource and a SciMesh Coordinator dashboard
(request rate & p95 by route, status mix, goroutines, RSS). Grafana allows
anonymous viewing so the dashboard opens without a login; admin/admin to edit.

- monitoring/prometheus.yml + grafana provisioning + dashboard JSON
- docker-compose.monitoring.yml overlay (third -f in demo-ui.sh)
- demo prints the Grafana and Prometheus URLs

coordinator/monitoring/prometheus.yml

@@ -0,0 +1,10 @@
+# Prometheus scrape config for the SciMesh demo. Prometheus runs in the same
+# compose network as the coordinator, so it reaches it by service name.
+global:
+  scrape_interval: 5s
+  evaluation_interval: 5s
+
+scrape_configs:
+  - job_name: coordinator
+    static_configs:
+      - targets: ["coordinator:8080"]