Create or update a user

Synchronously create or update a user in an environment. Requires the write:entities API key scope or admin member access. This scope also grants deletion through the corresponding entity DELETE endpoints, but does not grant membership changes or flag targeting; grant write:flag:targeting separately to assign flags and read for GET requests. Attributes accept JSON values and are normalized and merged using the same rules as ingestion. Recognized name, email, and avatar attributes also update the corresponding user fields; top-level name takes precedence over name attributes. Omitting a field preserves its existing value. New users may omit all fields. When this request succeeds, a subsequent user targeting request can read the user. Flag configuration propagation to SDKs is independent. Delayed ingestion writes may later replace overlapping values because the last database writer wins.