feat(train-scheduling): load returned empties onto an export train

Empties had no way onto a departure: the return record could name a train
but nothing seated it on a wagon. Export schedules now expose a loading
action that packs selected returns onto free wagons at one 40ft or two 20ft
each, enforced both in the picker and in the API (existing empties on the
schedule count against their wagon).

Adds container_size, train_schedule_id and wagon_sequence_no to
freight.empty_container_returns.

apps/edr-freight-api/src/modules/import-operations/empty-container-wagon.util.ts

@@ -0,0 +1,16 @@
+import { BadRequestException } from '@nestjs/common';
+
+/**
+ * A wagon carries ONE 40ft OR TWO 20ft empties — never a mix, never three.
+ * Throws on the first wagon that breaks the rule.
+ */
+export function assertWagonLoad(sizesByWagon: Map<number, string[]>): void {
+  for (const [wagon, sizes] of sizesByWagon) {
+    const has40 = sizes.some((size) => size === '40');
+    if ((has40 && sizes.length > 1) || sizes.length > 2) {
+      throw new BadRequestException(
+        `Wagon ${wagon} takes one 40ft or two 20ft containers — got ${sizes.join('ft + ')}ft`,
+      );
+    }
+  }
+}