Reject creating or editing a client with a subId already owned by a different client, mirroring the email-uniqueness checks against client_records in Create and Update (BulkCreate inherits via Create). The old multi-inbound model duplicated a client across inbounds sharing one subId, so this check was dropped; the first-class multi-client model makes per-client subId uniqueness correct again. Existing duplicates are left untouched; only new/edited duplicates are blocked.
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