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/components/warehouses/ReceiveInventoryModal.tsx

@@ -1414,8 +1414,30 @@ function ReadyToLoadTab({ enabled, onChanged }: { enabled: boolean; onChanged?:
   const { data: rows = [], isLoading } = useQuery(
     api.warehouses.readyToLoadExport.queryOptions({ enabled }),
   );
-  const loadPassed = useMutation(api.warehouses.loadPassedExport.mutationOptions());
+  const qc = useQueryClient();
   const [selected, setSelected] = useState<Set<string>>(new Set());
+  const [trainPickerOpen, setTrainPickerOpen] = useState(false);
+  const [targetScheduleId, setTargetScheduleId] = useState<string | null>(null);
+  // Loading is always onto a SPECIFIC pre-dispatch train. No train -> no auto-load.
+  const { data: trains = [], isLoading: trainsLoading } = useQuery({
+    queryKey: ['warehouse-inventory', 'loadable-trains'],
+    queryFn: () => warehouseService.getLoadableTrains(),
+    enabled: enabled && trainPickerOpen,
+  });
+  const loadOntoTrain = useMutation({
+    mutationFn: async (scheduleId: string) => {
+      const items = await warehouseService.getTrainLoadableItems(scheduleId);
+      const loadableIds = items.filter((i) => i.loadable).map((i) => i.id);
+      if (!loadableIds.length) {
+        throw new Error('No ready items with an allocated wagon on this train');
+      }
+      return warehouseService.loadItemsOntoTrain(scheduleId, loadableIds);
+    },
+    onSuccess: () => {
+      void qc.invalidateQueries({ queryKey: ['warehouse-inventory'] });
+      void qc.invalidateQueries({ queryKey: ['warehouse-loadings'] });
+    },
+  });
 
   const allSelected = rows.length > 0 && selected.size === rows.length;
   const someSelected = selected.size > 0 && !allSelected;
@@ -1427,13 +1449,22 @@ function ReadyToLoadTab({ enabled, onChanged }: { enabled: boolean; onChanged?:
       return next;
     });
 
-  const autoLoad = async () => {
+  const confirmLoad = async () => {
+    if (!targetScheduleId) {
+      toast({ variant: 'destructive', title: 'Select a train to load onto' });
+      return;
+    }
     try {
-      const r = await loadPassed.mutateAsync(undefined);
+      const r = await loadOntoTrain.mutateAsync(targetScheduleId);
+      const train = trains.find((t) => t.scheduleId === targetScheduleId);
       toast({
-        title: `${r.loadedCount} items loaded`,
-        description: r.skippedCount ? `${r.skippedCount} skipped` : undefined,
+        title: `${r.loadedCount} item(s) loaded onto train ${train?.trainNumber ?? ''}`.trim(),
+        description: r.skippedCount
+          ? `${r.skippedCount} skipped — ${r.results.find((x) => x.reason)?.reason ?? 'see results'}`
+          : undefined,
       });
+      setTrainPickerOpen(false);
+      setTargetScheduleId(null);
       setSelected(new Set());
       onChanged?.();
     } catch (error) {
@@ -1452,14 +1483,58 @@ function ReadyToLoadTab({ enabled, onChanged }: { enabled: boolean; onChanged?:
           variant="filled"
           color="teal"
           leftSection={<Truck size={14} />}
-          loading={loadPassed.isPending}
           disabled={rows.length === 0}
-          onClick={autoLoad}
+          onClick={() => setTrainPickerOpen(true)}
         >
           Auto Load Ready Items
         </Button>
       </Group>
 
+      <Modal
+        opened={trainPickerOpen}
+        onClose={() => setTrainPickerOpen(false)}
+        title="Load ready items onto a train"
+        centered
+        size="lg"
+      >
+        <Stack gap="md">
+          {trainsLoading ? (
+            <Group justify="center" py="md"><Loader size="sm" /></Group>
+          ) : trains.length === 0 ? (
+            <Alert color="yellow" variant="light" icon={<Info size={16} />}>
+              No train available. Auto-loading needs a scheduled (not yet dispatched) train with
+              these bookings assigned — schedule the train and allocate wagons first.
+            </Alert>
+          ) : (
+            <Select
+              label="Available trains"
+              placeholder="Select the train to load onto"
+              data={trains.map((t) => ({
+                value: t.scheduleId,
+                label: `${t.trainNumber ?? t.scheduleId.slice(0, 8)} · ${t.origin ?? '?'} → ${t.destination ?? '?'} · dep ${t.departureTime ? formatDate(t.departureTime) : '—'} · ${t.readyCount} ready`,
+              }))}
+              value={targetScheduleId}
+              onChange={setTargetScheduleId}
+              searchable
+            />
+          )}
+          <Group justify="flex-end">
+            <Button variant="default" onClick={() => setTrainPickerOpen(false)} disabled={loadOntoTrain.isPending}>
+              Cancel
+            </Button>
+            <Button
+              color="teal"
+              leftSection={<Truck size={14} />}
+              loading={loadOntoTrain.isPending}
+              disabled={!targetScheduleId}
+              onClick={confirmLoad}
+            >
+              Load onto this train
+            </Button>
+          </Group>
+        </Stack>
+      </Modal>
+
       {isLoading ? (
         <Group justify="center" py="lg">
           <Loader size="sm" />