feat: implement parity checks for 20ft container bookings to ensure even numbers

apps/edr-freight-api/src/modules/contracts/contract-booking.service.ts

@@ -2458,6 +2458,22 @@ export class ContractBookingService {
   private async assert20ftPairableAtCreate(
     dto: CreateBookingUnderContractDto,
   ): Promise<void> {
+    // Parity gate. 20ft containers ride two per wagon, so an odd total leaves
+    // one container that cannot be placed. Consolidation (pairing it with
+    // another customer's odd booking) is built end to end but switched off for
+    // now, so an odd total is rejected outright — server-side, because the
+    // frontend block alone is not a guarantee.
+    const ft20Quantity = (dto.containers ?? [])
+      .filter((line) => (line.containerSize ?? '').includes('20'))
+      .reduce((sum, line) => sum + Number(line.quantity || 0), 0);
+    if (ft20Quantity % 2 === 1) {
+      throw new BadRequestException(
+        `20ft containers travel two per wagon, so they must be booked in even ` +
+          `numbers. This booking has ${ft20Quantity} — add one more or remove ` +
+          `one (book ${ft20Quantity + 1} or ${ft20Quantity - 1}).`,
+      );
+    }
+
     const twentyFtUnits = (dto.containers ?? [])
       .filter((line) => (line.containerSize ?? '').includes('20'))
       .flatMap((line, lineIdx) =>