fix: backtest submission shows no results — 4 bugs fixed

- ID mismatch: API generated a throwaway UUID while BacktestReplay
  generated its own internally. Frontend polled with wrong ID and
  never found the DB row. Now pre-generate ID in endpoint and pass
  it to BacktestReplay.
- Field name: API returned 'backtest_id' but frontend read 'data.id'.
  Unified to 'id' everywhere.
- No polling: useBacktestResult fired once and never refreshed.
  Added refetchInterval that polls every 2s while status is running.
- Response shape: GET endpoint nested results under 'result' object
  but frontend expected flat fields. Flattened response to match
  BacktestResult type.
- Added running/failed/completed status indicators in BacktestPanel.

frontend/src/api/tradingHooks.ts

@@ -234,12 +234,17 @@ export function useTradingConfig() {
   });
 }
 
-/** Fetch a backtest result by ID. */
+/** Fetch a backtest result by ID. Polls every 2s while status is pending/running. */
 export function useBacktestResult(id: string | undefined) {
   return useQuery<BacktestResult>({
     queryKey: ['backtest-result', id],
     queryFn: () => apiGet<BacktestResult>('trading', `/api/trading/backtest/${id}`),
     enabled: !!id,
+    refetchInterval: (query) => {
+      const status = query.state.data?.status;
+      if (!status || status === 'running' || status === 'pending') return 2000;
+      return false;
+    },
   });
 }