nixos-module: add scheduler timer for artisan schedule:run

Invoice Ninja relies on Laravel's scheduler for recurring invoices,
scheduled reports, and other internal cron-driven jobs. Without a
systemd unit invoking `artisan schedule:run` periodically, the app's
dashboard reports "cron jobs are not enabled" after login.

Adds invoiceninja-scheduler.timer (every minute, matching upstream's
documented cron cadence) and its oneshot service, and extends the
automated test to assert the timer is active and schedule:run succeeds.

Note: the app's own is_scheduler_running flag only flips on a 5-minute
wall-clock boundary (app/Console/Kernel.php's everyFiveMinutes() job),
so the dashboard warning can take a few minutes to clear after boot -
confirmed working via the "Refresh Data" button in that same modal.

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

tests/nixos-test.nix

@@ -52,6 +52,11 @@ pkgs.testers.nixosTest {
     with subtest("invoiceninja-manage wrapper can run artisan commands"):
         machine.succeed("invoiceninja-manage migrate:status")
 
+    with subtest("scheduler timer is enabled and schedule:run succeeds"):
+        # Without this, the app's dashboard reports "cron jobs are not enabled".
+        machine.succeed("systemctl is-active invoiceninja-scheduler.timer")
+        machine.succeed("systemctl start --wait invoiceninja-scheduler.service")
+
     with subtest("admin account can log in via the API"):
         login_response = machine.succeed(
             "curl -sf -X POST http://localhost/api/v1/login "