feat: add hardened base and per-edition image overlays

Add the files that land in the image, organised as layered rootfs trees
(base first, then the edition layer wins on conflict).

- base: hardened kernel cmdline + sysctl, default-deny nftables,
  key-only ssh, persistent journald, module blacklist, no core dumps,
  snapshot timer units, motd.
- server: Prometheus + auto-provisioned Grafana + Loki + promtail.
- workstation: dev profile and desktop sysctl relaxations (perf,
  rootless containers).
- cloud: cloud-init provisioning config.

overlays/server/etc/grafana/provisioning/dashboards/node-overview.json

@@ -0,0 +1,94 @@
+{
+  "uid": "arcline-node-overview",
+  "title": "Arcline Node Overview",
+  "tags": ["arcline", "node"],
+  "timezone": "browser",
+  "schemaVersion": 39,
+  "version": 1,
+  "refresh": "30s",
+  "time": { "from": "now-1h", "to": "now" },
+  "panels": [
+    {
+      "id": 1,
+      "title": "System load",
+      "type": "timeseries",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 0 },
+      "datasource": { "type": "prometheus", "uid": "Prometheus" },
+      "targets": [
+        {
+          "expr": "node_load1",
+          "legendFormat": "load1",
+          "refId": "A"
+        },
+        {
+          "expr": "node_load5",
+          "legendFormat": "load5",
+          "refId": "B"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": { "unit": "short" },
+        "overrides": []
+      }
+    },
+    {
+      "id": 2,
+      "title": "Memory used",
+      "type": "timeseries",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 0 },
+      "datasource": { "type": "prometheus", "uid": "Prometheus" },
+      "targets": [
+        {
+          "expr": "(1 - node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes) * 100",
+          "legendFormat": "% used",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": { "unit": "percent", "max": 100, "min": 0 },
+        "overrides": []
+      }
+    },
+    {
+      "id": 3,
+      "title": "Disk usage",
+      "type": "gauge",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 8 },
+      "datasource": { "type": "prometheus", "uid": "Prometheus" },
+      "targets": [
+        {
+          "expr": "(1 - node_filesystem_avail_bytes{mountpoint=\"/\"} / node_filesystem_size_bytes{mountpoint=\"/\"}) * 100",
+          "legendFormat": "/",
+          "refId": "A"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": { "unit": "percent", "max": 100, "min": 0 },
+        "overrides": []
+      }
+    },
+    {
+      "id": 4,
+      "title": "Network traffic",
+      "type": "timeseries",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 8 },
+      "datasource": { "type": "prometheus", "uid": "Prometheus" },
+      "targets": [
+        {
+          "expr": "rate(node_network_receive_bytes_total[5m])",
+          "legendFormat": "rx {{ $labels.device }}",
+          "refId": "A"
+        },
+        {
+          "expr": "rate(node_network_transmit_bytes_total[5m])",
+          "legendFormat": "tx {{ $labels.device }}",
+          "refId": "B"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": { "unit": "Bps" },
+        "overrides": []
+      }
+    }
+  ]
+}