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.Domain/Backtests/Backtest.cs

@@ -41,6 +41,7 @@ public class Backtest
     public string ScoreMessage { get; set; } = string.Empty;
     [Required] public decimal InitialBalance { get; set; }
     [Required] public decimal NetPnl { get; set; }
+    [Required] public int PositionCount { get; set; }
 
     /// <summary>
     /// Creates a new TradingBotConfig based on this backtest's configuration for starting a live bot.

src/Managing.Domain/Backtests/LightBacktest.cs

@@ -27,4 +27,5 @@ public class LightBacktest
     [Id(14)] public string Ticker { get; set; } = string.Empty;
     [Id(15)] public decimal InitialBalance { get; set; }
     [Id(16)] public decimal NetPnl { get; set; }
+    [Id(17)] public int PositionCount { get; set; }
 }