fix(tours): keep the city walk helpers only their own files use inside them (CW-01, CW-03, CW-04)

The filter for a walk's stations in the planner, the opening windows of a
walk's schedule and the station shape of a walk's progress were exported,
although nothing outside their own file reads them, so the check for unused
exports counted three more than it allows. They are local to their files now;
a city walk plans, checks off and times its stops as before.

server/src/nest/tours/tour-visits.service.ts

@@ -111,7 +111,7 @@ export class TourVisitsService {
 }
 
 /** CW-09: one station of a walk on a day, as an assistant reads it. */
-export interface WalkProgressStation {
+interface WalkProgressStation {
   position: number;
   name: string | null;
   lat: number;