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/test/mocks/handlers.ts

@@ -219,6 +219,15 @@ export const handlers = [
   http.get('/api/agents/:agent_id/variants/:variant_id/performance', () => HttpResponse.json(mockVariantPerformance)),
   http.get('/api/agents/:agent_id/variants/:variant_id/performance/history', () => HttpResponse.json(mockVariantPerfHistory)),
 
+  // Trading Engine: Override order
+  http.post('/trading/api/trading/override/order', async ({ request }) => {
+    const body = await request.json() as Record<string, unknown>;
+    return HttpResponse.json(
+      { job_id: 'job-test-123', status: 'queued', ticker: body.ticker, side: body.side, quantity: body.quantity, auto_registered: false },
+      { status: 202 },
+    );
+  }),
+
   // Competitive intelligence endpoints
   http.get('/registry/companies/:id/competitors', () => HttpResponse.json(mockCompetitors)),
   http.post('/registry/companies/:id/competitors/infer', () => HttpResponse.json(mockCompetitors)),