Rename ElementId to Id in Signature record

The `ElementId` property in the `Signature` record was renamed to `Id`. This change simplifies the property name, making it more concise and aligning with standard naming conventions or domain terminology.

EnvelopeGenerator.Application/Common/Dto/Signature.cs

@@ -15,7 +15,7 @@ public sealed record Signature
     /// <summary>
     /// TBDD_DOCUMENT_RECEIVER_ELEMENT.ID - identifies the specific signature field on the PDF page.
     /// </summary>
-    public required int ElementId { get; init; }
+    public required int Id { get; init; }
 
     /// <summary>
     /// Base64-encoded data URL of the signature image.