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/dto/load-inventory.dto.ts

@@ -6,6 +6,11 @@ export class LoadInventoryDto {
   @IsUUID()
   wagonId!: string;
 
+  @ApiPropertyOptional({ format: 'uuid', description: 'Train schedule this load belongs to (recorded on the loading).' })
+  @IsOptional()
+  @IsUUID()
+  trainScheduleId?: string;
+
   @ApiPropertyOptional({ description: 'Weight loaded onto the wagon (t)' })
   @IsOptional()
   @IsNumber()