Links two or more client records together (e.g. a company and its beneficial owners). Pass an array of relation items where each side (from, to) is identified by spektrId or reference. Each item must specify relationRole (one of the supported role keys such as beneficial_owners, shareholders, ultimate_beneficial_owners, senior_management, board_of_directors, controlling_entities, subsidiaries, authorized_signatories, signatories, contact_info, person_with_significant_control) and may include optional data metadata, positions, and a direct boolean. The legacy roles array is no longer accepted - send relationRole directly instead. The request is atomic: all relations are created together or none are, if any validation fails. Validation rules: both records must exist, from and to cannot be the same record, duplicate relations are rejected, and the resulting graph must remain acyclic.