feat: port tauri create-service payment flow to admin session settlement

Port the tauri /files/create-service page (payment mode) to the admin SPA
and back it with real multi-part session settlement:

Backend:
- New SessionPayment entity (session_payments table): partial payments
  per session with method (wallet/pos/cash/card), amount, paid_at, actor
- PatientSession: settlement discount (percent/fixed), discount_rials,
  paid_at, payments relation; remaining debt derived from
  final - discount - paid total
- POST /api/v1/session/{uuid}/payments: register a partial payment;
  wallet method debits the patient wallet; zero remaining marks paid
- PATCH /api/v1/session/{uuid}: accepts discount_type/discount_value
  (null removes) and paid_at, backward compatible
- New error codes: ERR_SESSION_PAYMENT_INVALID/_EXCEEDS,
  ERR_SESSION_DISCOUNT_INVALID
- Migration + 14 functional tests (partial/full/wallet/exceed/discount)

Frontend (admin):
- SessionPaymentPage: two-step stepper (پرداخت ← جزییات) ported from
  tauri AddService payment mode — service cost, settlement discount
  input, Jalali payment date, wallet balance, 4-method payment accordion,
  paid-list box, details summary
- SessionStepper + stepper/payment icons ported verbatim from tauri SVGs
- «تکمیل پرداخت» on SessionServiceCard now navigates to the payment page
  (replaces the small settle modal on PatientDetailPage)
- Routes for patients/ and my-patients/ variants; vitest coverage
- docs/api/patient.md updated

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

assets/admin/App.tsx

@@ -42,6 +42,7 @@ import MyPatientsPage from './pages/MyPatientsPage';
 import MyPaymentsPage from './pages/MyPaymentsPage';
 import MyPaymentDetailPage from './pages/MyPaymentDetailPage';
 import NewSessionPage from './pages/NewSessionPage';
+import SessionPaymentPage from './pages/SessionPaymentPage';
 import InsurancePricingPage from './pages/InsurancePricingPage';
 import ClaimsPage from './pages/ClaimsPage';
 import DoctorClaimsPage from './pages/DoctorClaimsPage';
@@ -211,6 +212,8 @@ export default function App() {
         <Route path="patients/:uuid" element={<RoleRoute roles={['doctor', 'secretary', 'clinic']} blockClinicScope><PatientDetailPage /></RoleRoute>} />
         <Route path="patients/:recordUuid/session/new" element={<RoleRoute roles={['doctor', 'secretary', 'clinic']} blockClinicScope><NewSessionPage /></RoleRoute>} />
         <Route path="my-patients/:recordUuid/session/new" element={<RoleRoute roles={['doctor', 'secretary', 'clinic']} blockClinicScope><NewSessionPage /></RoleRoute>} />
+        <Route path="patients/:recordUuid/session/:sessionUuid/pay" element={<RoleRoute roles={['doctor', 'secretary', 'clinic']} blockClinicScope><SessionPaymentPage /></RoleRoute>} />
+        <Route path="my-patients/:recordUuid/session/:sessionUuid/pay" element={<RoleRoute roles={['doctor', 'secretary', 'clinic']} blockClinicScope><SessionPaymentPage /></RoleRoute>} />
         <Route path="insurance-pricing" element={<RoleRoute roles={['doctor', 'clinic']} blockClinicScope><InsurancePricingPage /></RoleRoute>} />
         <Route path="claims" element={<RoleRoute roles={['doctor', 'clinic']} blockClinicScope><ClaimsPage /></RoleRoute>} />
         <Route path="my-financial" element={<RoleRoute roles={['doctor', 'secretary', 'clinic']}><MyFinancialPage /></RoleRoute>} />