Signs a shared review aggregate on behalf of the caller, as the reviewee or as the manager. Required scope: w_reviews. Requires a user token, or an account token with member_id, whose employee is the reviewee or the aggregate's manager (an account token without member_id gets 403). No request body.

The cycle must have signing enabled (a sign deadline) and must not be archived; the aggregate must already be shared and not yet signed by the caller; signing is rejected after the sign deadline and, when the cycle requires an employee meeting, until a meeting is scheduled. The aggregate becomes signed once both the reviewee and the manager have signed.