For a firm: Change a client's name, reference, rules or active flag (billing, admin)

Operator mount: an operator acting for the firm named in the path, as its oldest active admin, else its oldest active billing member; X-Six-Minute-Acting-As names them. 404 for no such firm, 409 urn:six-minute:problem:no-acting-member for one with neither. Served for an internal tenant alone: any other is 409 urn:six-minute:problem:tenant-not-internal, before the audit row and before any write.