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/default.nix

@@ -1,8 +1,18 @@
 { lib, pkgs, config, ... }:
 let
-  serviceDownAlertRules = (import ./service_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
+  giteaDownAlertRules = (import ./gitea_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
+  giteaRunnerDownAlertRules = (import ./gitea_runner_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
+  postfixDownAlertRules = (import ./postfix_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
+  dovecotDownAlertRules = (import ./dovecot_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
+  openldapDownAlertRules = (import ./openldap_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
+  wireguardDownAlertRules = (import ./wireguard_down.nix { inherit lib pkgs config; }).grafanaAlertRuleDefinitions;
 
-  allServiceRules = serviceDownAlertRules;
+  allServiceRules = giteaDownAlertRules
+                   ++ giteaRunnerDownAlertRules
+                   ++ postfixDownAlertRules
+                   ++ dovecotDownAlertRules
+                   ++ openldapDownAlertRules
+                   ++ wireguardDownAlertRules;
 in
 {
   services.grafana.provision.alerting.rules.settings.groups = [