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/RouteController.cs

@@ -31,6 +31,13 @@ public class RouteController(RouteService service) : ControllerBase
         return result is null ? NotFound() : Ok(result);
     }
 
+    [HttpPut("{id:guid}/region")]
+    public async Task<IActionResult> AssignToRegion(Guid id, AssignRouteToRegionRequest request, CancellationToken ct)
+    {
+        var result = await service.AssignToRegionAsync(id, request, ct);
+        return result is null ? NotFound() : Ok(result);
+    }
+
     [HttpDelete("{id:guid}")]
     public async Task<IActionResult> Delete(Guid id, CancellationToken ct)
     {