fix(mcp-gateway): change to user systemd service

Use systemd.user.services instead of systemd.services, default config
to ~/.config/mcp-gateway/gateway.yaml, and wantedBy default.target.

flake.nix

@@ -55,6 +55,10 @@
     // {
       packages = inputs.nixpkgs.lib.recursiveUpdate blueprintOutputs.packages extraPackages;
 
+      nixosModules = {
+        mcp-gateway = ./modules/mcp-gateway.nix;
+      };
+
       overlays = {
         default = import ./overlays {
           inherit (blueprintOutputs) packages;

modules/mcp-gateway.nix

@@ -0,0 +1,46 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.mcp-gateway;
+  package = pkgs.mcp-gateway;
+in
+
+{
+  options.services.mcp-gateway = {
+    enable = lib.mkEnableOption "MCP Gateway user service";
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = package;
+      description = "The mcp-gateway package to use.";
+    };
+
+    configFile = lib.mkOption {
+      type = lib.types.str;
+      default = "%h/.config/mcp-gateway/gateway.yaml";
+      description = "Path to the mcp-gateway configuration file.";
+    };
+
+    settings = lib.mkOption {
+      type = with lib.types; attrsOf (attrsOf anything);
+      default = {};
+      description = "Extra systemd service configuration options.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.user.services.mcp-gateway = {
+      description = "MCP Gateway";
+      wantedBy = [ "default.target" ];
+      wants = [ "network-online.target" ];
+      after = [ "network-online.target" ];
+
+      serviceConfig = {
+        Type = "simple";
+        ExecStart = "${cfg.package}/bin/mcp-gateway --config ${cfg.configFile} serve";
+        Restart = "on-failure";
+        RestartSec = 5;
+      } // cfg.settings;
+    };
+  };
+}