Add dedicated endpoint to assign a route to a region

PUT /api/routes/{id}/region assigns/reassigns an existing route to a
region directly, validating the target region exists and is active.
Stands in for the blocked assignAreaToRegion until an Area entity
exists between Region and Route.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Distribution/Features/Route/RouteDTOs.cs

@@ -5,3 +5,5 @@ public record CreateRouteRequest(Guid RegionId, string RouteName, string StartPo
 public record UpdateRouteRequest(Guid RegionId, string RouteName, string StartPoint, string EndPoint, decimal Length);
 
 public record RouteResponse(Guid Id, Guid RegionId, string RouteName, string StartPoint, string EndPoint, decimal Length, bool IsActive, DateTime CreatedAt);
+
+public record AssignRouteToRegionRequest(Guid RegionId);