add keepalive to system config for uptime kuma monitoring

modules/system/keepalive/default.nix

@@ -0,0 +1,46 @@
+{
+  pkgs,
+  config,
+  lib,
+  ...
+}:
+let
+  service = "keepalive";
+  cfg = config.system.${service};
+in {
+  options.system.${service} = {
+    enable = lib.mkEnableOption "enables ${service}";
+    url = lib.mkOption {
+      type = lib.types.str;
+      default = "";
+      description = "url to ping for uptime-kuma keepalive";
+    };
+    interval = lib.mkOption {
+      type = lib.types.str;
+      default = "60s";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.services.keepalive = {
+      description = "send push for uptime kuma heartbeat";
+      serviceConfig = {
+        Type = "oneshot";
+      };
+      script = ''
+        ${pkgs.curl}/bin/curl -fsSk \
+          "${cfg.url}" \
+          >/dev/null
+      '';
+    };
+
+    systemd.timers.keepalive = {
+      wantedBy = [ "timers.target" ];
+      timerConfig = {
+        OnBootSec = "30s";
+        OnUnitActiveSec = cfg.interval;
+        Unit = "keepalive.service";
+      };
+    };
+  };
+}