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.

services/trading/backtest_replay.py

@@ -39,16 +39,18 @@ class BacktestReplay:
         self.pool = pool
         self._perf = PerformanceComputer()
 
-    async def run(self, config: BacktestConfig) -> BacktestResult:
+    async def run(self, config: BacktestConfig, backtest_id: str | None = None) -> BacktestResult:
         """Execute a full backtest replay.
 
         Args:
             config: Backtest configuration (date range, capital, risk tier).
+            backtest_id: Optional pre-generated ID. If not provided, one is generated.
 
         Returns:
             BacktestResult with metrics, trade log, and equity curve.
         """
-        backtest_id = str(uuid.uuid4())
+        if backtest_id is None:
+            backtest_id = str(uuid.uuid4())
 
         try:
             # Fetch historical recommendations