feat: add Prometheus + Grafana + Portainer, remove Netdata

- Replace Netdata with Prometheus + Node Exporter + cAdvisor + Grafana
- Add Portainer CE for container management
- metrics.carloselugo.com now serves Grafana
- portainer.carloselugo.com added behind internal_only snippet
- Prometheus internal only, no public port exposed
- Retention: 15d
- Dashboards: Node Exporter Full (1860), cAdvisor (14282)

prometheus/prometheus.yml

@@ -0,0 +1,16 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  - job_name: "prometheus"
+    static_configs:
+      - targets: ["localhost:9090"]
+
+  - job_name: "node-exporter"
+    static_configs:
+      - targets: ["node-exporter:9100"]
+
+  - job_name: "cadvisor"
+    static_configs:
+      - targets: ["cadvisor:8080"]