feat: override trade tab — manual order entry with auto-registration

Backend:
- OverrideOrderRequest/Response Pydantic models with ticker, quantity, price validators
- POST /api/trading/override/order endpoint (enqueue to Redis broker queue)
- auto_register_symbol() module for untracked ticker registration via Symbol Registry
- Unit tests (17) and property-based tests (3 x 100 examples)

Frontend:
- OverrideTradePanel component (order form + positions display)
- Override tab in TradingEngine page with URL search param navigation
- Override Trade button on Trading Controls page
- useSubmitOverrideOrder mutation hook
- MSW handler and 13 component/integration tests

Steering:
- Updated steering docs for Ubuntu dev machine with nvm/Node 24

frontend/src/routes.tsx

@@ -115,6 +115,9 @@ const tradingEngineRoute = createRoute({
   getParentRoute: () => rootRoute,
   path: '/trading/engine',
   component: TradingEnginePage,
+  validateSearch: (search: Record<string, unknown>): { tab?: string } => ({
+    tab: typeof search.tab === 'string' ? search.tab : undefined,
+  }),
 });
 const opsPipelineRoute = createRoute({
   getParentRoute: () => rootRoute,