Phase 3b: Repository-Schicht im Core (Interfaces + Mongo-Impl + DI)

- Interfaces: IAccountRepository, IMarketRepository, IPositionRepository
  (IPositionRepository kapselt das Collection-per-Account-Muster).
- Mongo-Implementierungen bilden die bisherige Shim-Semantik 1:1 mit direkten
  MongoDB.Driver-Aufrufen nach (kein App-Shim-Bezug im Core).
- AddCorePersistence()-Erweiterung + Registrierung in Program.cs.
- Noch keine Call-Sites umgestellt; Build 0 Fehler.

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

Program.cs

@@ -8,6 +8,7 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Options;
 using PolyTrader.Core.Configuration;
+using PolyTrader.Core.DependencyInjection;
 using PolyTraderSharp.Models;
 using PolyTraderSharp.Services;
 
@@ -33,6 +34,7 @@ internal static class Program
 					var client = new MongoClient(dbOptions.ConnectionString);
 					return client.GetDatabase(dbOptions.DatabaseName);
 				});
+				services.AddCorePersistence();
 				services.AddSingleton((IServiceProvider sp) => ServerSettings.Load("server_settings.xml"));
 				services.AddSingleton<TradingState>();
 				services.AddSingleton(copySignalChannel.Writer);