feat: store payment method before preorder pricing

Reject pricing until the customer picks cash or check terms, then open that payment path after the invoice.

Co-authored-by: Cursor <cursoragent@cursor.com>

prisma/migrations/20261115100000_preorder_payment_plan/migration.sql

@@ -0,0 +1,2 @@
+-- روش پرداخت و مدت چک قبل از قیمت‌گذاری روی سفارش ذخیره می‌شود
+ALTER TABLE "Order" ADD COLUMN "checkTermMonths" INTEGER;