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/system/host_down.nix

@@ -0,0 +1,54 @@
+{ lib, pkgs, config, ... }:
+{
+  grafanaAlertRuleDefinitions = [
+    {
+      uid = "host-down-alert-uid";
+      title = "HostDown";
+      condition = "C";
+
+      data = [
+        {
+          refId = "A";
+          datasourceUid = "vm-datasource-uid";
+          queryType = "prometheus";
+          relativeTimeRange = { from = 60; to = 0; }; # Query over the last minute
+          model = {
+            expr = ''up'';
+            legendFormat = "{{instance}} ({{job}})";
+            instant = false; # Changed from true, as relativeTimeRange is used
+          };
+        }
+        { # New Expression B: Reduce Query A
+          refId = "B";
+          datasourceUid = "__expr__";
+          model = {
+            type = "reduce";
+            expression = "A"; # Input is Query A
+            reducer = "last"; # Get the last value of each series in A
+          };
+        }
+        { # Modified Expression C: Math condition based on B
+          refId = "C";
+          datasourceUid = "__expr__";
+          model = {
+            type = "math";
+            expression = "$B == 0"; # Check if the last value from B is 0
+          };
+        }
+      ];
+
+      for = "2m";
+      noDataState = "Alerting";
+      execErrState = "Error";
+
+      annotations = {
+        summary = "Host {{ $labels.instance }} is down";
+        description = ''Host {{ $labels.instance }} (job: {{ $labels.job }}) has been down for more than 2 minutes.'';
+      };
+      labels = {
+        severity = "critical";
+        category = "availability";
+      };
+    }
+  ];
+}