feat(customers): hard-delete customers

DELETE /customers/<id> now removes the row.

- Deletes layaways and dependent items/payments.

- Nullifies customer_id in sales, service_orders, fleet_vehicles, plate_vehicles, quotations, returns, warranties, bnpl_transactions, customer_payments.

- Cascade-delete CRM tables are handled by FK constraints.

pos/static/js/customers.js

@@ -442,7 +442,7 @@ const Customers = (() => {
             alert('No tienes permiso para eliminar clientes');
             return;
         }
-        if (!confirm(`¿Eliminar al cliente "${currentCustomer.name}"? Se marcará como inactivo.`)) return;
+        if (!confirm(`¿Eliminar al cliente "${currentCustomer.name}"? Se borrará completamente. Las ventas, órdenes y vehículos conservarán sus datos pero quedarán sin cliente asignado.`)) return;
         try {
             await api(`/pos/api/customers/${currentCustomer.id}`, { method: 'DELETE' });
             alert('Cliente eliminado');