List external users

List this app's managed end-users. This is the runtime-key view a developer dashboard or coding agent can use after users arrive through ``POST /v1/sessions`` or explicit upserts. It is app-scoped, returns the developer-controlled ``external_user_ref`` as the stable identifier, and includes budget/usage/session summary fields. It does not expose raw conversations; use session/event APIs for a specific user's sessions.