Phase 6 (Stufe 2): Core-EF-Repos + umschaltbare Registrierung

- EF-Implementierungen hinter den Core-Interfaces: EfAccountRepository,
  EfMarketRepository, EfPositionRepository, EfTradeLogRepository. Thread-safe via
  IDbContextFactory<CoreDbContext> (kurzlebiger Context je Operation).
- AddCorePersistence(DatabaseOptions): Provider "MySql" -> EF + DbContextFactory,
  "Mongo" -> Mongo-Repos. Program.cs uebergibt die aufgeloesten Optionen.
- appsettings.json Provider-Default = "Mongo" (App bleibt vorerst auf MongoDB,
  kein Verhaltenswechsel). MySQL-Connection liegt in gitignorierter appsettings.Local.json.
- Build 0 Fehler.

Naechste Stufen: Modul-Context + EF-Repos (inkl. trackers/mt_history), Config-Migration
Mongo->MySQL, dann Provider umschalten + Mongo entfernen.

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

src/PolyTrader.Core/Persistence/Ef/EfAccountRepository.cs

@@ -0,0 +1,42 @@
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.EntityFrameworkCore;
+using PolyTraderSharp.Models;
+
+namespace PolyTrader.Core.Persistence.Ef
+{
+    public class EfAccountRepository : IAccountRepository
+    {
+        private readonly IDbContextFactory<CoreDbContext> _factory;
+
+        public EfAccountRepository(IDbContextFactory<CoreDbContext> factory) => _factory = factory;
+
+        public List<AccountState> GetAll()
+        {
+            using var ctx = _factory.CreateDbContext();
+            return ctx.Accounts.AsNoTracking().ToList();
+        }
+
+        public void Upsert(AccountState account)
+        {
+            using var ctx = _factory.CreateDbContext();
+            var existing = ctx.Accounts.Find(account.AccountId);
+            if (existing == null)
+                ctx.Accounts.Add(account);
+            else
+                ctx.Entry(existing).CurrentValues.SetValues(account);
+            ctx.SaveChanges();
+        }
+
+        public void Delete(int accountId)
+        {
+            using var ctx = _factory.CreateDbContext();
+            var existing = ctx.Accounts.Find(accountId);
+            if (existing != null)
+            {
+                ctx.Accounts.Remove(existing);
+                ctx.SaveChanges();
+            }
+        }
+    }
+}