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-web/backoffice/src/services/api.ts

@@ -101,7 +101,6 @@ import type {
   InitiateWarehouseInvoicePaymentPayload,
   LoadableWagon,
   LoadInventoryPayload,
-  LoadPassedExportResult,
   MoveInventoryPayload,
   StoreInventoryPayload,
   PayInvoicePayload,
@@ -1158,14 +1157,6 @@ export const api = {
       () => INVENTORY_INVALIDATIONS,
     ),
 
-    loadPassedExport: endpoint<void, LoadPassedExportResult>(
-      "warehouse-inventory",
-      "load-passed-export",
-      () => warehouseService.loadPassedExport().then((r) => r.data),
-      undefined,
-      () => INVENTORY_INVALIDATIONS,
-    ),
-
     bulkMarkInspected: endpoint<BulkInspectPayload, BulkInspectResult>(
       "warehouse-inventory",
       "bulk-mark-inspected",