TodosService should not pluck IDs

app/services/todos_service.rb has the following code:

todos.select(&:id)

This violates the rule to not load IDs when passing the values directly to a sub-query: https://docs.gitlab.com/ee/development/sql.html#plucking-ids. Here select(&:id) is basically the same as pluck(:id).

This code should instead use a sub-query.