ZARINPAL_REDIRECT_URL was empty on the server, so after a real (non-sandbox)
payment the callback answered with {"orderId","status"} on the API domain and
the student was stranded there. It now defaults to FRONTEND_URL
(https://math.tadrisino2.ir/) with ?orderId=&status=; reopening a finished
order's pay link goes back to the app the same way. "-" keeps the old JSON.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>