feat(monitoring): deploy Prometheus + Grafana stack via Docker

- prometheus.yml: scrapes node, postgres, redis, nexus-pos, nexus-quart
- docker-compose.monitoring.yml: Prometheus, Grafana, node-exporter,
  postgres-exporter, redis-exporter
- Grafana auto-provisions Prometheus datasource
- Access: Grafana :3001 (admin/nexus2026), Prometheus :9090

docker/grafana/provisioning/datasources/prometheus.yml

@@ -0,0 +1,9 @@
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: false