Returns the full customer object identified by objectId. The retailer is resolved from the
authorizer token; the repository validates that the customer belongs to the authenticated retailer.

Returns 404 for both non-existent and cross-tenant object IDs to prevent information disclosure.

The response includes maskedBirthDate (format **/**/YYYY when birthDate is present),
address wrapped as an array (legacy compatibility — L-004), and salesPerson projected
with selected fields. All three are omitted when absent on the customer record.