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.Application/Envelopes/Commands/CreateEnvelopeCommand.cs

@@ -33,7 +33,18 @@ public record CreateEnvelopeCommand : IRequest<EnvelopeDto?>
     /// <summary>
     /// ID des Absenders
     /// </summary>
-    public int UserId { get; set; }
+    internal int UserId { get; private set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="userId"></param>
+    /// <returns></returns>
+    public bool Authorize(int userId)
+    {
+        UserId = userId;
+        return true;
+    }
 
     /// <summary>
     /// Determines which component is used for envelope processing.