G3 & G4: Map UsdcSize, OutcomeIndex, compute Category ROI and add migration

src/Predictalytics.Domain/Entities/Trade.cs

@@ -66,6 +66,13 @@ public class Trade
     /// <summary>Transaction hash (for blockchain-based platforms).</summary>
     public string? TransactionHash { get; set; }
 
+    /// <summary>USDC size of the trade (raw currency size traded).</summary>
+    [Column(TypeName = "decimal(18,6)")]
+    public decimal? UsdcSize { get; set; }
+
+    /// <summary>Outcome index traded.</summary>
+    public int? OutcomeIndex { get; set; }
+
     // ── Context Enrichment (AI Strategy Detection) ───────────
     
     /// <summary>Market price 1 minute before trade execution.</summary>