feat: implement centralized alerting with vmalert and Grafana, add alert rules for CPU, disk, inode, RAM usage, and host status

hosts/web-arm/modules/vmalert/rules/host_down.nix

@@ -0,0 +1,23 @@
+{ lib, pkgs, config, ... }: # Standard module arguments
+{
+  services.vmalert.rules.groups = [
+    {
+      name = "HostStatusAlerts";
+      rules = [
+        {
+          alert = "HostDown";
+          expr = "up == 0";
+          for = "2m";
+          labels = {
+            severity = "critical";
+            category = "availability";
+          };
+          annotations = {
+            summary = "Host {{ $labels.instance }} is down";
+            description = "Host {{ $labels.instance }} (job: {{ $labels.job }}) has been down for more than 2 minutes.";
+          };
+        }
+      ];
+    }
+  ];
+}