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/ram_usage.nix

@@ -0,0 +1,23 @@
+{ lib, pkgs, config, ... }: # Standard module arguments
+{
+  services.vmalert.rules.groups = [
+    {
+      name = "RAMUsageAlerts";
+      rules = [
+        {
+          alert = "HighRAMUsage";
+          expr = "(1 - node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes) * 100 > 90";
+          for = "10m";
+          labels = {
+            severity = "warning";
+            category = "performance";
+          };
+          annotations = {
+            summary = "High RAM usage on {{ $labels.instance }}";
+            description = "RAM usage on {{ $labels.instance }} (job: {{ $labels.job }}) has been above 90% for more than 10 minutes. Current value: {{ $value | printf \"%.2f\" }}%.";
+          };
+        }
+      ];
+    }
+  ];
+}