Add carrier field to Refund model and update Stripe views

Introduces a OneToOneField for carrier in the Refund model to support future integration and refactoring. Adds and updates TODO comments for email template usage, error handling in Stripe webhooks, and clarifies logic placement in Stripe models.

backend/thirdparty/stripe/views.py

@@ -26,12 +26,13 @@ class StripeWebhook(APIView):
         sig_header = request.META['HTTP_STRIPE_SIGNATURE']
 
         try:
-            #build stripe event
+            #build stripe event (api call)
             event = stripe.Webhook.construct_event(
                 payload, sig_header, os.getenv("STRIPE_WEBHOOK_SECRET")
             )
 
         except ValueError as e:
+            # TODO: doupravit error handling, dodělat v modelu možnost pro uložení chyby a status ERROR
             logger.error(f"Invalid payload: {e}")
             return HttpResponse(status=400)