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>

tests/common.nix

@@ -0,0 +1,15 @@
+{ ... }:
+
+{
+  imports = [ ../nixos-module/invoiceninja.nix ];
+
+  services.invoiceninja = {
+    enable = true;
+    proxy.server = "nginx";
+    adminAccount.passwordFile = ./invoice_ninja_test_password;
+    secretFile = ./test.env;
+    settings.APP_DEBUG = true;
+  };
+
+  networking.firewall.enable = false;
+}