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/EnvelopeController.cs

@@ -90,19 +90,18 @@ public class EnvelopeController : ControllerBase
     /// <summary>
     /// 
     /// </summary>
-    /// <param name="envelope"></param>
+    /// <param name="command"></param>
     /// <returns></returns>
     [NonAction]
     [Authorize]
     [HttpPost]
-    public async Task<IActionResult> CreateAsync([FromQuery] CreateEnvelopeCommand envelope)
+    public async Task<IActionResult> CreateAsync([FromBody] CreateEnvelopeCommand command)
     {
-        envelope.UserId = User.GetId();
-        var res = await _mediator.Send(envelope);
+        var res = await _mediator.Send(command.Authorize(User.GetId()));
 
         if (res is null)
         {
-            _logger.LogError("Failed to create envelope. Envelope details: {EnvelopeDetails}", JsonConvert.SerializeObject(envelope));
+            _logger.LogError("Failed to create envelope. Envelope details: {EnvelopeDetails}", JsonConvert.SerializeObject(command));
             return StatusCode(StatusCodes.Status500InternalServerError);
         }
         else