add Excel import functionality for container bookings

apps/edr-freight-api/src/modules/train-scheduling/booking-split.service.ts

@@ -36,6 +36,9 @@ export interface SizedOffer {
  * rows, so reducing the lines releases it automatically) and can be rebooked in
  * any later window within contract validity. A ONE_TIME contract is promoted to
  * GENERAL on split (see applySplit) so its remainder is actually rebookable.
+ * Once the remainder is rebooked and the cap hits zero, ContractBookingService
+ * completes the contract (CONTRACT_CLOSED): no further bookings or shipment
+ * requests, even while validity and a booking window are still open.
  */
 @Injectable()
 export class BookingSplitService {