Refactor controllers for MediatR and cleaner API design

- Switch EnvelopeController and ReceiverController to MediatR for all operations
- Encapsulate UserId in CreateEnvelopeCommand via Authorize() method
- Change CreateEnvelopeCommand binding to [FromBody]
- Add CancellationToken support to EnvelopeReceiverController
- Remove obsolete CRUD logic from ReceiverController; now only supports GET via MediatR
- Clean up unused dependencies and update controller summaries for clarity

EnvelopeGenerator.GeneratorAPI/Controllers/EnvelopeReceiverController.cs

@@ -105,6 +105,7 @@ public class EnvelopeReceiverController : ControllerBase
     /// Datenübertragungsobjekt mit Informationen zu Umschlägen, Empfängern und Unterschriften.
     /// </summary>
     /// <param name="request"></param>
+    /// <param name="cancel"></param>
     /// <returns>HTTP-Antwort</returns>
     /// <remarks>
     /// Sample request:
@@ -140,13 +141,10 @@ public class EnvelopeReceiverController : ControllerBase
     /// <response code="500">Es handelt sich um einen unerwarteten Fehler. Die Protokolle sollten überprüft werden.</response>
     [Authorize]
     [HttpPost]
-    public async Task<IActionResult> CreateAsync([FromBody] CreateEnvelopeReceiverCommand request)
+    public async Task<IActionResult> CreateAsync([FromBody] CreateEnvelopeReceiverCommand request, CancellationToken cancel)
     {
-        CancellationToken cancel = default;
-        int userId = User.GetId();
-
         #region Create Envelope
-        var envelope = await _envelopeExecutor.CreateEnvelopeAsync(userId, request.Title, request.Message, request.TFAEnabled, cancel);
+        var envelope = await _envelopeExecutor.CreateEnvelopeAsync(User.GetId(), request.Title, request.Message, request.TFAEnabled, cancel);
         #endregion
 
         #region Add receivers