build(nix): add Nix flake, NixOS module, and README documentation

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

nix/module.nix

@@ -0,0 +1,49 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let cfg = config.services.zugferd-service;
+in {
+  options.services.zugferd-service = {
+    enable = mkEnableOption "ZUGFeRD REST API service";
+
+    port = mkOption {
+      type = types.port;
+      default = 5000;
+      description = "Port to listen on";
+    };
+
+    host = mkOption {
+      type = types.str;
+      default = "127.0.0.1";
+      description = "Host to bind to";
+    };
+
+    package = mkOption {
+      type = types.package;
+      default = pkgs.zugferd-service;
+      description = "Package to use for the service";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.zugferd-service = {
+      description = "ZUGFeRD REST API Service";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network.target" ];
+
+      serviceConfig = {
+        Type = "simple";
+        ExecStart =
+          "${cfg.package}/bin/zugferd-service --host ${cfg.host} --port ${
+            toString cfg.port
+          }";
+        Restart = "on-failure";
+        DynamicUser = true;
+        NoNewPrivileges = true;
+        ProtectSystem = "strict";
+        ProtectHome = true;
+      };
+    };
+  };
+}