feat(warehouses): auto-load targets a selected train, never loads trainless

"Auto Load Ready Items" now opens a train picker (pre-dispatch trains with
these bookings assigned, via the existing loadable-trains flow). No train
available -> no auto-loading, with a clear notice. Loading goes through the
existing per-wagon load path, so items without an allocated wagon are skipped
with a reason.

The train association is stored on the existing warehouse_loadings table
(no new table needed): new train_schedule_id column + a note recording train
number, origin -> destination, and departure time; wagon_id becomes nullable.
The trainless load-passed-export endpoint, its frontend wiring, and the unused
useLoadPassedExport hook are removed.

Migration 2100000000000 (idempotent) also applied to the dev database.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/edr-freight-api/src/modules/warehouses/entities/warehouse-loading.entity.ts

@@ -28,9 +28,17 @@ export class WarehouseLoading extends BaseEntity {
   @JoinColumn({ name: 'booking_id' })
   booking?: Booking | null;
 
-  /** Physical wagon the item was loaded onto. References freight.wagons (read-only link). */
-  @Column({ name: 'wagon_id', type: 'uuid' })
-  wagonId!: string;
+  /**
+   * Physical wagon the item was loaded onto. References freight.wagons
+   * (read-only link). Nullable: a schedule-level auto-load may not resolve to
+   * one wagon — the train association then lives in trainScheduleId.
+   */
+  @Column({ name: 'wagon_id', type: 'uuid', nullable: true })
+  wagonId?: string | null;
+
+  /** Train schedule the item was loaded onto (read-only link to scheduling). */
+  @Column({ name: 'train_schedule_id', type: 'uuid', nullable: true })
+  trainScheduleId?: string | null;
 
   @Column({ name: 'loaded_at', type: 'timestamptz' })
   loadedAt!: Date;