send the student back to the web app after paying, not to a raw JSON page

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>

payment/views.py

@@ -201,6 +201,11 @@ class PaymentRedirectView(APIView):
 
         if order.status != 'pending':
             # سفارش از قبل نتیجه گرفته (paid/failed)؛ چیزی برای پرداختِ تازه نیست.
+            # کاربر به اپ برمی‌گردد با همان نتیجه، نه به ریشه‌ی دامنه‌ی API.
+            if settings.ZARINPAL_REDIRECT_URL:
+                return _redirect_to_configured_url(
+                    f'{settings.ZARINPAL_REDIRECT_URL}?orderId={order.id}&status={order.status}'
+                )
             return HttpResponseRedirect(settings.PUBLIC_BASE_URL or '/')
 
         if not order.gateway_authority: