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.

tests/test_pbt_trading_api.py

@@ -222,7 +222,7 @@ class TestProperty29PersistenceRoundTrip:
         assert isinstance(resp.json()["ready"], bool)
 
     def test_backtest_returns_id(self) -> None:
-        """POST /api/trading/backtest returns a backtest_id string."""
+        """POST /api/trading/backtest returns an id string."""
         client = _get_client()
         resp = client.post(
             "/api/trading/backtest",
@@ -235,9 +235,9 @@ class TestProperty29PersistenceRoundTrip:
         )
         assert resp.status_code == 200
         data = resp.json()
-        assert "backtest_id" in data
-        assert isinstance(data["backtest_id"], str)
-        assert len(data["backtest_id"]) > 0
+        assert "id" in data
+        assert isinstance(data["id"], str)
+        assert len(data["id"]) > 0
 
     def test_backtest_get_returns_placeholder(self) -> None:
         """GET /api/trading/backtest/{id} returns a result dict."""
@@ -246,7 +246,7 @@ class TestProperty29PersistenceRoundTrip:
         resp = client.get(f"/api/trading/backtest/{test_id}")
         assert resp.status_code == 200
         data = resp.json()
-        assert data["backtest_id"] == test_id
+        assert data["id"] == test_id
 
     def test_decisions_returns_list(self) -> None:
         """GET /api/trading/decisions returns a list."""