Create a reviewer.

Adds one reviewer account, with the same rules as adding a reviewer from the Users list in the AwardSpring admin interface: a first name, a last name and a well-formed email are required, each at most 50 characters. As in the admin interface, a new reviewer gets no welcome email.

An email that already has an account: by default the email must not already be used by any account at the institution, ignoring upper and lower case; if it is, this returns 400 email_already_exists. With if_email_in_use set to add_role, this does what the admin interface does instead, and no second account is made. When an active account at the institution has the email, the reviewer role is added to that account, which keeps its name, email and password, and this returns 200 with that account as the reviewer. An account that already has the reviewer role is returned unchanged, also with 200. The role is never added to an account that holds an administrator role, and this returns 400 email_already_exists instead, as it does for an email whose account is deactivated, merged, or one the admin interface would not change. When only an archived account has the email, that account gets no role: a new reviewer is made with 201 and the email moves to them, as in the admin interface. The AwardSpring-Existing-Account header says which of these happened: role_added, role_already_held or email_moved_from_archived. It is absent for an ordinary create, and from a response replayed for an Idempotency-Key.

Idempotency: the request supports an optional Idempotency-Key header (any printable-ASCII string, 1–255 chars — UUIDs work well). When supplied, the same key plus the same request body within 24 hours returns the original response verbatim without creating a duplicate reviewer. The same key with a different body returns 422 idempotency_key_request_mismatch; a concurrent retry while the first call is still executing returns 409 idempotency_key_in_flight.