feat(services): add systemd email notification module

Introduce a NixOS module for sending service status emails on failures via systemd. The module provides configurable options for email recipients and integrates with the `sendmail` utility to notify admins about service issues.

modules/nixos/services/systemd-email-notify/default.nix

@@ -0,0 +1,73 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+with lib;
+
+let
+  checkConditions = pkgs.writeScript "checkConditions" ''
+    #!/bin/sh
+    STATUS=$(systemctl status --full "$1")
+
+    case "$STATUS" in
+      *"activating (auto-restart) (Result: timeout)"*) exit 1 ;;
+      *) exit 0 ;;
+    esac
+  '';
+
+  sendmail = pkgs.writeScript "sendmail" ''
+    #!/bin/sh
+
+    ${pkgs.system-sendmail}/bin/sendmail -t <<ERRMAIL
+    To: $1
+    From: ${config.systemd.email-notify.mailFrom}
+    Subject: Status of service $2
+    Content-Transfer-Encoding: 8bit
+    Content-Type: text/plain; charset=UTF-8
+
+    $(systemctl status --full "$2")
+    ERRMAIL
+  '';
+in
+{
+  options = {
+    systemd.email-notify.mailTo = mkOption {
+      type = types.str;
+      default = "admin";
+      description = "Email address to which the service status will be mailed.";
+    };
+
+    systemd.email-notify.mailFrom = mkOption {
+      type = types.str;
+      default = "admin";
+      description = "Email address from which the service status will be mailed.";
+    };
+
+    systemd.services = mkOption {
+      type =
+        with types;
+        attrsOf (submodule {
+          config.onFailure = [ "email@%n.service" ];
+        });
+    };
+  };
+
+  config = {
+    systemd.services."email@" = {
+      description = "Sends a status mail via sendmail on service failures.";
+      onFailure = mkForce [ ];
+      unitConfig = {
+        StartLimitIntervalSec = "5m";
+        StartLimitBurst = 1;
+      };
+      serviceConfig = {
+        ExecCondition = "${checkConditions} %i";
+        ExecStart = "${sendmail} ${config.systemd.email-notify.mailTo} %i";
+        Type = "oneshot";
+      };
+    };
+  };
+}