Archive a person outside the school matched by the partner reference (externalReferenceId) written on the record. Same effects, errors and webhook as DELETE /externals/:id.

The reference is not unique in legacy data, so the rule is: no record carries it → 404 EXTERNAL_NOT_FOUND; only archived records carry it → 409 EXTERNAL_ALREADY_ARCHIVED; exactly one ACTIVE record carries it → that record is archived (archived records sharing the reference never block); two or more ACTIVE records carry it → 409 EXTERNAL_REFERENCE_NOT_UNIQUE, nothing is archived and no webhook is sent: errors[] lists every active record (context.externalId, most recently edited first), so the caller archives the right one with DELETE /externals/:id or gives each record its own reference with PATCH /externals/:id. A reference longer than 128 characters answers 400 VALIDATION_ERROR.