refactor: Grafana alerting rules: consolidate and reorganize alert definitions

- Deleted individual alert files for host down, inode usage, and RAM usage.
- Merged service down alerts into a new structure with separate files for each service (Gitea, Postfix, Dovecot, OpenLDAP, WireGuard).
- Introduced a new system alert structure consolidating CPU, disk, host down, inode, and RAM usage alerts.
- Updated alert conditions to use 'D' for thresholds and adjusted expressions accordingly.
- Improved annotations and labels for clarity and consistency across alerts.

hosts/web-arm/modules/grafana/alerting/service/wireguard_down.nix

@@ -0,0 +1,57 @@
+{ lib, pkgs, config, ... }:
+{
+  grafanaAlertRuleDefinitions = [
+    {
+      uid = "wireguard-service-down-alert-uid";
+      title = "WireGuard Service Down";
+      condition = "C";
+      data = [
+        {
+          refId = "A";
+          relativeTimeRange = {
+            from = 300;
+            to = 0;
+          };
+          datasourceUid = "vm-datasource-uid";
+          model = {
+            editorMode = "code";
+            expr = "node_systemd_unit_state{state=\"active\", name=\"wireguard-wg_cloonar.service\"} OR on() vector(0)";
+            hide = false;
+            intervalMs = 1000;
+            legendFormat = "__auto";
+            maxDataPoints = 43200;
+            range = true;
+            refId = "A";
+          };
+        }
+        {
+          refId = "B";
+          datasourceUid = "__expr__";
+          model = {
+            type = "reduce";
+            expression = "A";
+            reducer = "last";
+          };
+        }
+        {
+          refId = "C";
+          datasourceUid = "__expr__";
+          model = {
+            type = "math";
+            expression = "$B < 1";
+          };
+        }
+      ];
+      noDataState = "Alerting";
+      execErrState = "Alerting";
+      for = "5m";
+      annotations = {
+        description = "WireGuard service is down on {{ $labels.instance }}";
+        summary = "WireGuard Service Down";
+      };
+      labels = {
+        severity = "critical";
+      };
+    }
+  ];
+}