Permanently removes the customer identified by objectId (hard delete — no soft delete or deletedAt flag).

The retailer is resolved from the authorizer token; a tenant check via findById is performed before deletion.
Returns 404 for both non-existent and cross-tenant object IDs to prevent information disclosure.

This operation is irreversible. Related objects (Address, CustomFieldValue) are not removed and
remain as orphans in this version. A second DELETE on the same objectId returns 404 (non-idempotent).