refactor(envelope): consolidate draft flow into upsert command

- extend CreateEnvelopeCommand to support draft updates and send intent

- merge draft save logic into CreateEnvelopeCommandHandler and remove duplicate handler

- add DocReceiverElement writer abstraction and infrastructure registration for SQL-based field upserts

EnvelopeGenerator.Application/Common/Interfaces/IDocReceiverElementWriter.cs

@@ -0,0 +1,10 @@
+using EnvelopeGenerator.Domain.Entities;
+
+namespace EnvelopeGenerator.Application.Common.Interfaces;
+
+public interface IDocReceiverElementWriter
+{
+    Task DeleteByDocumentAndReceiverAsync(int documentId, int receiverId, CancellationToken cancellationToken = default);
+
+    Task InsertAsync(DocReceiverElement element, CancellationToken cancellationToken = default);
+}