feat: comprehensive observability stack

- Add restic_backups and mattermost scrape jobs to Prometheus config
- Add UnPoller deployment+service+secret for UniFi metrics (unpoller.yaml)
- Add unifi scrape job targeting unpoller in-cluster
- Add 4 Grafana dashboards as NixOS-managed etc files on hosea:
  - system-health: CPU, memory, disk, network per host
  - backup-health: restic backup freshness and duration
  - kubernetes: pod counts, restart rates, node resources
  - network: DNS queries, DHCP leases, UniFi ports, ping latency
- Add grafana-api-token agenix secret placeholder on hosea
- Open firewall ports 9100/9427/9558 for prometheus exporters in baseline.nix

manifests/monitoring/unpoller.yaml

@@ -0,0 +1,65 @@
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: unpoller-credentials
+  namespace: monitoring
+type: Opaque
+stringData:
+  # Replace with actual UniFi controller password for the 'unpoller' read-only user
+  password: "CHANGEME"
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: unpoller
+  namespace: monitoring
+  labels:
+    app: unpoller
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: unpoller
+  template:
+    metadata:
+      labels:
+        app: unpoller
+    spec:
+      containers:
+        - name: unpoller
+          image: ghcr.io/unpoller/unpoller:latest
+          ports:
+            - name: metrics
+              containerPort: 9130
+          env:
+            - name: UP_UNIFI_DEFAULT_URL
+              value: "https://10.42.1.1:8443"
+            - name: UP_UNIFI_DEFAULT_VERIFY_SSL
+              value: "false"
+            - name: UP_UNIFI_DEFAULT_USER
+              value: "unpoller"
+            - name: UP_UNIFI_DEFAULT_PASS
+              valueFrom:
+                secretKeyRef:
+                  name: unpoller-credentials
+                  key: password
+            - name: UP_PROMETHEUS_DISABLE
+              value: "false"
+            - name: UP_INFLUXDB_DISABLE
+              value: "true"
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: unpoller
+  namespace: monitoring
+  labels:
+    app: unpoller
+spec:
+  selector:
+    app: unpoller
+  ports:
+    - name: metrics
+      port: 9130
+      targetPort: 9130