Add automated NixOS VM test and CLAUDE.md

Adds a pkgs.testers.nixosTest (checks.<system>.invoiceninja) that boots
the module end-to-end and checks that mysql/redis/phpfpm/queue-worker/
nginx come up, the one-time migrate/seed/admin-create setup completed,
the app shell and /login page are served, invoiceninja-manage works,
and the seeded admin can log in via the API.

Factors the shared services.invoiceninja config used by both the
interactive VM and the new test into tests/common.nix, and satisfies
NixOS's fileSystems/bootloader assertions in the interactive VM config
so `nix flake check` evaluates nixosConfigurations.test cleanly (the
qemu-vm module already overrides both when actually building
config.system.build.vm, so this only affects the toplevel-eval path).

Also adds a `just test` recipe, a CLAUDE.md codebase overview, and a
.gitignore for the result/nixos.qcow2 build artifacts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flake.nix

@@ -32,6 +32,10 @@
         default = pkgsBySystem.${system}.invoiceninja;
       });
 
+      checks = forEachSystem (system: {
+        invoiceninja = import ./tests/nixos-test.nix { inherit system; nixpkgsPath = nixpkgs; };
+      });
+
       nixosModules = import ./nixos-module { overlays = overlayList; };
     };
 }