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>

docs/ADMIN.md

@@ -314,6 +314,27 @@ container/facade/service split as the rest of `admin/*`:
 - Wired into `/​:lang/backoffice/orders` and `/​:lang/backoffice/orders/:id`,
   replacing the coming-soon placeholder.
 
+## Sprint 24 - Transactions (mock/local)
+
+`features/admin/transactions/`. `AdminTransactionsLocalGateway` derives its
+mock data from `AdminOrdersLocalGateway`'s 24 seeded orders (one
+transaction per order, deterministic type/status/method assignment) rather
+than a separate synthetic dataset - keeps order numbers/totals consistent
+between the two mock feature areas.
+
+- List: search, status filter, type filter (payment/refund/qr_payment),
+  pagination, CSV export.
+- Retry failed transactions (`status: 'failed' -> 'retried'`, appends an
+  audit entry).
+- Fraud flag toggle per transaction.
+- Audit log: each transaction carries its own `audit: AdminTransactionAuditEntry[]`
+  (creation, retries, fraud-flag changes), viewed via a dialog - this is a
+  per-transaction audit trail, not the system-wide audit/security log
+  planned for Sprint 26 (Monitoring); the two are intentionally separate
+  scopes.
+- Wired into `/​:lang/backoffice/transactions`, replacing the coming-soon
+  placeholder.
+
 ## Known gaps / backend needs
 
 - **Dashboard metrics endpoint.** Categories/Products counts are computed