feat(reports): record loading and unloading time per stop

The OCC report publishes total loading and unloading time and the other
activity left over from a station stay, but nothing recorded when handling
started or ended — the July 2026 seed had to write the figure into a
checkpoint note. Four nullable stamps now ride the stop's arrival row, which
is the row the staying-time report builds a stay from (a turnaround's
departure belongs to a different schedule).

Handling is unloading start to loading end, so a container stop reads as one
window and a bulk station that only loads or only unloads still reports its
half; other activity is the rest of the stay. Both stay NULL where nothing
was logged rather than collapsing to zero.

- station-staying-time: + loading/unloading and other activity per stop
- turnaround-cycle: + the same, summed over the cycle's stops
- loading-unloading (new): per train per station per period, so a week or
  month view is that train's average over its stops
- the stop/stay query moves to operations-classification, shared by both

packages/types/src/freight/index.ts

@@ -370,6 +370,11 @@ export interface ITrainCheckpointEvent extends BaseEntity {
   sequenceNo: number;
   kind: TrainCheckpointKind;
   occurredAt: string;
+  /** Station work during the stay this stop opens. Null = never logged. */
+  unloadingStartedAt?: string | null;
+  unloadingCompletedAt?: string | null;
+  loadingStartedAt?: string | null;
+  loadingCompletedAt?: string | null;
   note?: string | null;
   recordedByUserId?: string | null;
 }