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/default.nix

@@ -29,11 +29,6 @@ in
 {
   imports = [
     # Individual alert files removed, now handled by alerting/system/default.nix
-    # ./alerting/disk_usage.nix
-    # ./alerting/cpu_usage.nix
-    # ./alerting/host_down.nix
-    # ./alerting/inode_usage.nix
-    # ./alerting/ram_usage.nix
     ./alerting/system/default.nix # Added: Imports the consolidated system alerts module
     ./alerting/service/default.nix # Added: Imports the new service alerts module
     # ... other rule files can be added here ...