Phase 5.3-ClosedTrade: ClosedTrade -> Modul + ICopyTradeLogRepository

- ClosedTrade (+ ClosedTradeRow) nach src/PolyTrader.Modules.CopyTrading/Models/.
- Neues ICopyTradeLogRepository (+ Mongo-Impl) im Modul: EnsureIndexes, Exists,
  Insert, Find(predicate) auf der closed_trades-Collection.
- closed_trades-Zugriffe der Modul-Services vom App-Shim auf das Repo umgestellt:
  TraderMonitorService, PersistenceService, PolymarketWssClient, TraderAnalyticsJob.
  Shim-/DB-Usings dort entfernt.
- TraderMonitorService nutzt kein _db mehr (auch die uebrigen _db-Guards aus 3d
  entfernt, da Repos immer verfuegbar sind). Verhaltensneutral.
- frm_main (UI) + Program.cs (BsonDocument-Cleanup) bleiben auf _db.
- Build 0 Fehler.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Models/ClosedTrade.cs

@@ -1,36 +0,0 @@
-namespace PolyTraderSharp.Models
-{
-    public class ClosedTrade
-    {
-        [MongoDB.Bson.Serialization.Attributes.BsonId] public int TradeId { get; set; }
-        public int AccountId { get; set; }
-        public int SourceTraderId { get; set; }
-        public bool IsDemo { get; set; }
-
-        public string TokenId { get; set; } = string.Empty;
-        public string MarketSlug { get; set; } = string.Empty;
-        public string MarketQuestion { get; set; } = string.Empty;
-        public string Outcome { get; set; } = string.Empty;
-
-        public string Side { get; set; } = string.Empty;
-        public decimal EntryPrice { get; set; }
-        public decimal ExitPrice { get; set; }
-        public decimal Size { get; set; }
-
-        public decimal RealizedPnl { get; set; }
-        public decimal PnlPercent { get; set; }
-        public decimal TotalFees { get; set; }
-
-        public DateTime OpenedAt { get; set; }
-        public DateTime ClosedAt { get; set; }
-        public string ExitReason { get; set; } = string.Empty;
-    }
-    public class ClosedTradeRow : ClosedTrade
-    {
-        public string AccountName { get; set; } = string.Empty;
-        public string SourceTraderName { get; set; } = string.Empty;
-        
-        [System.ComponentModel.Browsable(false)]
-        public string SourceTraderAddress { get; set; } = string.Empty;
-    }
-}