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/prometheus/prometheus.yml

@@ -0,0 +1,30 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  - job_name: 'prometheus'
+    static_configs:
+      - targets: ['localhost:9090']
+
+  - job_name: 'node'
+    static_configs:
+      - targets: ['node-exporter:9100']
+
+  - job_name: 'postgres'
+    static_configs:
+      - targets: ['postgres-exporter:9187']
+
+  - job_name: 'redis'
+    static_configs:
+      - targets: ['redis-exporter:9121']
+
+  - job_name: 'nexus-pos'
+    static_configs:
+      - targets: ['host.docker.internal:5001']
+    metrics_path: /metrics
+
+  - job_name: 'nexus-quart'
+    static_configs:
+      - targets: ['host.docker.internal:5002']
+    metrics_path: /metrics