Add PositionCount property to Backtest models and responses

- Introduced PositionCount to Backtest, LightBacktest, and their respective response models.
- Updated BacktestController and BacktestExecutor to include PositionCount in responses.
- Modified database schema to accommodate new PositionCount field in relevant entities.

src/Managing.Application/Backtests/BacktestExecutor.cs

@@ -410,6 +410,7 @@ public class BacktestExecutor
             EndDate = candles.LastOrDefault()!.OpenTime,
             InitialBalance = initialBalance,
             NetPnl = netPnl, // Net PnL after fees
+            PositionCount = tradingBot.Positions.Count,
         };
 
         if (save && user != null)
@@ -526,7 +527,8 @@ public class BacktestExecutor
             Score = backtest.Score,
             ScoreMessage = backtest.ScoreMessage,
             InitialBalance = backtest.InitialBalance,
-            NetPnl = backtest.NetPnl
+            NetPnl = backtest.NetPnl,
+            PositionCount = backtest.PositionCount
         };
     }