two things:

1. `last_updated_at` isn't good for a search query -- really you want to get all records `updated_after`
2. do you agree that folks will not normally be combining the `updated_after` keyword with other search terms?  Apps using that keyword are just trying to get all updates.  Since those updates will soon be using some higher form of transaction isolation that we might not want to impose on all user searches, maybe the `updated_after` query should be its own dedicated endpoint (not just an index method).  E.g. `/api/application_users/updated_after`.  Thoughts?