GET /clients/{clientId}/account-summary

A client's whole-account money view (ITE-656): what they owe (outstanding AR across open invoices), what the practice holds for them (store credit), and the open invoices behind the headline number. Rows reuse the clinic-wide AR row shape (OutstandingInvoiceDto, ITE-317) — per-invoice Balance derives from the same settled-payments sum as the headline, so list and headline cannot disagree (the ITE-307 lesson).