Creates draft purchase orders to restock the given stock records (variant-at-location rows, as returned by the stock endpoints). Every stock must be at or below its reorder limit and its product must have a supplier, otherwise the call fails with 400. Stocks are grouped by location and supplier — one draft PO per group — and each line's quantity is reorder_limit - current count in the variant's default unit, costed at the variant's current cost price at the location. Requires the create-purchase-orders permission at each location.