feat(admin): transaction management

Sprint 24.

New features/admin/transactions/ module. AdminTransactionsLocalGateway
derives one synthetic transaction per Sprint 23's seeded mock order rather
than a separate dataset, keeping order numbers/totals consistent across
the two mock feature areas.

- list: search, status filter, type filter (payment/refund/qr_payment),
  pagination, CSV export
- retry failed transactions (appends an audit entry)
- fraud flag toggle
- per-transaction audit log (creation/retry/fraud-flag-change), viewed via
  dialog - intentionally separate from the system-wide audit/security log
  planned for Sprint 26 (Monitoring)
- wired into /:lang/backoffice/transactions, replacing the coming-soon
  placeholder

docs/ADMIN.md + docs/BACKEND.md (new item 13) updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/app.routes.ts

@@ -93,8 +93,7 @@ const coreRoutes: Routes = [
       },
       {
         path: 'transactions',
-        loadComponent: () => import('./features/backoffice/shared/backoffice-coming-soon-page.component').then(m => m.BackofficeComingSoonPageComponent),
-        data: { titleKey: 'dashboard.actionTransactions' }
+        loadComponent: () => import('./features/admin/transactions/pages/admin-transactions-list-page.component').then(m => m.AdminTransactionsListPageComponent)
       },
       {
         path: 'orders',