Add Nix packaging with NixOS module

- nix/package.nix: derivation for personal-backlog server + webapp
- nix/module.nix: NixOS module for services.personal-backlog
- Expose packages and nixosModules in flake.nix

Usage in NixOS configuration:
  services.personal-backlog = {
    enable = true;
    port = 8080;
    dataDir = "/var/lib/personal-backlog";
    openFirewall = false;
  };

flake.nix

@@ -18,17 +18,18 @@
         "x86_64-linux"
       ];
 
+      flake = {
+        nixosModules.default = import ./nix/module.nix;
+        nixosModules.personal-backlog = import ./nix/module.nix;
+      };
+
       perSystem =
         {
           system,
+          pkgs,
           ...
         }:
         let
-          pkgs = import nixpkgs {
-            inherit system;
-            config.allowUnfree = true;
-          };
-
           pre-commit-check = git-hooks.lib.${system}.run {
             src = ./.;
             hooks = {
@@ -39,6 +40,11 @@
         {
           formatter = pkgs.nixfmt-tree;
 
+          packages = {
+            default = pkgs.callPackage ./nix/package.nix { };
+            personal-backlog = pkgs.callPackage ./nix/package.nix { };
+          };
+
           checks = {
             inherit pre-commit-check;
           };