- 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