Phase 4.7: MullvadVpnService + ThreemaService in den Core

- Beide Services nach Core (nutzen nur TerminalLogger/ServerSettings/JobManager,
  keine DB/TradingState/Modul-Typen).
- Threema-ProjectReference von App -> Core umgehängt (App nutzte die Lib nur
  über ThreemaService).
- Microsoft.Extensions.Hosting.Abstractions als Core-Paket (BackgroundService).
- Toten Stub services/mullvad.cs gelöscht; Mullvad-DB-Usings entfernt.
- Build 0 Fehler.

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

PolyTrader.App.csproj

@@ -75,7 +75,6 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="libs\Threema-MsgApi-Net-Core\IcgSoftware.Threema.CoreMsgApi\IcgSoftware.Threema.CoreMsgApi.csproj" />
     <ProjectReference Include="src\PolyTrader.Core\PolyTrader.Core.csproj" />
     <ProjectReference Include="src\PolyTrader.Modules.CopyTrading\PolyTrader.Modules.CopyTrading.csproj" />
   </ItemGroup>

services/mullvad.cs

@@ -1,18 +0,0 @@
-using System;
-using MongoDB.Driver;
-using PolyTraderSharp.Extensions;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
-namespace PolyTraderSharp.services
-{    /*
-     * Verwaltet alles was mit Mullvad zu tun hat. Also VPN verbindung aktivieren / deaktivieren / prüfen
-     */
-    internal class Mullvad
-    {
-
-    }
-
-}

src/PolyTrader.Core/PolyTrader.Core.csproj

@@ -10,7 +10,12 @@
     <PackageReference Include="MongoDB.Driver" Version="2.24.0" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="8.0.0" />
     <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Hosting.Abstractions" Version="8.0.0" />
     <PackageReference Include="Nethereum.Web3" Version="6.1.0" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="..\..\libs\Threema-MsgApi-Net-Core\IcgSoftware.Threema.CoreMsgApi\IcgSoftware.Threema.CoreMsgApi.csproj" />
+  </ItemGroup>
+
 </Project>

src/PolyTrader.Core/Services/MullvadVpnService.cs

@@ -1,6 +1,4 @@
 using System;
-using MongoDB.Driver;
-using PolyTraderSharp.Extensions;
 using System.Diagnostics;
 using System.IO;
 using System.Threading;