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

src/Predictalytics.Domain/Entities/Trader.cs

@@ -66,4 +66,5 @@ public class Trader
     public TraderScore? CurrentScore { get; set; }
     public virtual TraderAnalytics? Analytics { get; set; }
     public ICollection<WatchlistEntry> WatchlistEntries { get; set; } = new List<WatchlistEntry>();
+    public ICollection<TraderPosition> Positions { get; set; } = new List<TraderPosition>();
 }

src/Predictalytics.Domain/Entities/TraderPosition.cs

@@ -0,0 +1,34 @@
+using System;
+
+namespace Predictalytics.Domain.Entities;
+
+/// <summary>
+/// Represents a trader's position in a specific market outcome.
+/// Tracks shares held, average purchase price, and realized profit/loss.
+/// </summary>
+public class TraderPosition
+{
+    public int Id { get; set; }
+
+    /// <summary>Foreign key to the trader.</summary>
+    public int TraderId { get; set; }
+
+    /// <summary>Foreign key to the resolved market outcome.</summary>
+    public int MarketOutcomeId { get; set; }
+
+    /// <summary>Number of shares currently held.</summary>
+    public decimal SharesHeld { get; set; }
+
+    /// <summary>Average purchase cost per share.</summary>
+    public decimal AvgCost { get; set; }
+
+    /// <summary>Realized profit/loss from closed portions of this position.</summary>
+    public decimal RealizedPnl { get; set; }
+
+    /// <summary>When this position was last updated.</summary>
+    public DateTime LastUpdatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public Trader Trader { get; set; } = null!;
+    public MarketOutcome MarketOutcome { get; set; } = null!;
+}