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/pages/PatientDetailPage.test.tsx

@@ -199,16 +199,18 @@ describe('PatientDetailPage (پرونده تب‌دار)', () => {
     await waitFor(() => expect(post).toHaveBeenCalledWith('/api/v1/patient/r1/wallet/charge', { amount_rials: 500000, payment_method: 'cash' }));
   });
 
-  it('settles a session from the wallet via the payment-method chooser', async () => {
-    const patch = api.patch as ReturnType<typeof vi.fn>;
-    patch.mockResolvedValue({ success: true, data: {} });
-    renderDetail();
+  it('navigates to the session payment page when «تکمیل پرداخت» is clicked', async () => {
+    renderWithProviders(
+      <Routes>
+        <Route path="/admin/patients/:uuid" element={<PatientDetailPage />} />
+        <Route path="/admin/patients/:recordUuid/session/:sessionUuid/pay" element={<div>صفحه تکمیل پرداخت</div>} />
+      </Routes>,
+      { route: '/admin/patients/r1' },
+    );
     await loaded();
-    // تب سرویس‌ها پیش‌فرض است؛ کارت پرداخت‌نشده → «تکمیل پرداخت»
+    // تب سرویس‌ها پیش‌فرض است؛ کارت پرداخت‌نشده → «تکمیل پرداخت» → صفحه‌ی استپر پرداخت
     fireEvent.click(await screen.findByText('تکمیل پرداخت'));
-    expect(await screen.findByText('روش پرداخت مراجعه')).toBeInTheDocument();
-    fireEvent.click(screen.getByRole('button', { name: 'کیف پول بیمار' }));
-    await waitFor(() => expect(patch).toHaveBeenCalledWith('/api/v1/session/s1', { payment_method: 'wallet' }));
+    expect(await screen.findByText('صفحه تکمیل پرداخت')).toBeInTheDocument();
   });
 
   it('renders the notes tab with a compose box and empty state', async () => {