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/pages/Trading.tsx

@@ -1,4 +1,5 @@
 import { useState } from 'react';
+import { Link } from '@tanstack/react-router';
 import {
   useTradingConfig,
   useSetTradingMode,
@@ -78,6 +79,14 @@ export function TradingPage() {
               {mode}
             </button>
           ))}
+          <Link
+            to="/trading/engine"
+            search={{ tab: 'override' }}
+            className="rounded-md bg-brand-600 px-4 py-2 text-sm font-medium text-white hover:bg-brand-700"
+            data-testid="override-trade-button"
+          >
+            Override Trade
+          </Link>
         </div>
 
         {/* Confirmation dialog for live mode */}