Refactor EnvelopeDto and improve response handling

Updated the `EnvelopeDto` class by removing several properties to simplify the data structure and added `AddedWhen`, `ChangedWhen`, and a nullable `EnvelopeType`.

Modified the return statement in `EnvelopeController.cs` to return `NotFound()` when no envelopes are present, enhancing the accuracy of HTTP responses.

EnvelopeGenerator.GeneratorAPI/Controllers/EnvelopeController.cs

@@ -74,7 +74,7 @@ public class EnvelopeController : ControllerBase
                     if (envelope.Uuid is string uuid)
                         envelopes = envelopes.Where(e => e.Uuid == uuid);
 
-                    return Ok(envelopes);
+                    return envelopes.Any() ? Ok(envelopes) : NotFound();
                 },
                 Fail: IActionResult (msg, ntc) =>
                 {