Updates existing customers. Each item is identified by id or, if id is absent, by email (at least one is required, CUS0027). Only the provided fields are updated.

When id is provided, email updates the customer email unless it already exists on the site (silently kept otherwise).

Per-item result: success {customerId, customerEmail, success: true}, error {customerId, customerEmail, customerFidelity, success: false, errors} (HTTP 200).