Implement A1, A6, A2: Add TraderPosition entity, PositionPnLEngine (Average-Cost-Method), Market-level WinRate, and integrate with TraderAnalyticsWorker

src/Predictalytics.Application/Interfaces/IPositionPnLEngine.cs

@@ -0,0 +1,14 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Predictalytics.Application.Interfaces;
+
+public interface IPositionPnLEngine
+{
+    /// <summary>
+    /// Recalculates all positions, average costs, and PnL values for a trader by re-playing their trade history.
+    /// Updates the TraderPosition records in the database, calculates realized/unrealized PnL,
+    /// and updates the Trader's TotalPnl and WinRate.
+    /// </summary>
+    Task RecalculateTraderPositionsAsync(int traderId, CancellationToken ct = default);
+}