Port the remaining gap of tauri /files/invoice-summary into the admin
invoice summary: GET /api/v1/billing/invoices/{uuid} now includes a
'session' key (full PatientSession payload) so InvoiceSummaryModal can
render the consumables table, the itemized payments table (method,
amount, date-time, recorder) and real discount/paid/remaining figures
instead of heuristics. Invoices without a source session keep the
previous behavior (session: null).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>