(backend) wip introduce a token exchange endpoint

src/backend/core/urls.py

@@ -48,6 +48,11 @@ external_router.register(
     external_viewsets.RoomViewSet,
     basename="external_room",
 )
+external_router.register(
+    "users",
+    external_viewsets.UserViewSet,
+    basename="external_user",
+)
 
 urlpatterns = [
     path(