List App User Candidates

Who this app's admin may still grant access to: the app's ORG members.

Exactly the set add_app_user below accepts, minus whoever already holds a grant — so every name the picker offers is one the POST will take.

The UI used to build this list from GET /users, which is platform-admin only and returns every user on the platform. For an ordinary org owner that call 403s and the dropdown rendered empty with no error, so nobody outside platform staff could add a user to an app at all; for platform staff it listed every customer's address on the platform and most of them would have been refused on submit. Neither half was right.