feat: add metrics exporters for Dovecot and Postfix, update Signal execution command, and improve configuration management

hosts/mail/modules/metrics/dovecot-exporter.nix

@@ -0,0 +1,15 @@
+{ config, pkgs, lib, ... }:
+
+{
+  services.prometheus.exporters.dovecot = {
+    enable = true;
+  };
+
+  services.victoriametrics.extraScrapeConfigs = [
+    ''
+      - job_name: "dovecot-exporter"
+        static_configs:
+        - targets: ['localhost:9166']
+    ''
+  ];
+}