Remove EnvelopeSigningType enum and update envelope logic

Removed EnvelopeSigningType enum and related normalization logic. Added a ReadOnly property to Envelope that uses EnvelopeTypeId to determine read-only status. Envelope type handling now relies on EnvelopeTypeId (int?) instead of the enum.

EnvelopeGenerator.Domain/Constants/EnvelopeSigningType.cs

@@ -1,14 +0,0 @@
-namespace EnvelopeGenerator.Domain.Constants
-{
-    public enum EnvelopeSigningType
-    {
-        WetSignature = 1,
-        ReadAndSign = 2
-    }
-
-    public static class EnvelopeSigningTypeExtensions
-    {
-        public static EnvelopeSigningType Normalize(this EnvelopeSigningType value) =>
-            value == EnvelopeSigningType.ReadAndSign ? EnvelopeSigningType.ReadAndSign : EnvelopeSigningType.WetSignature;
-    }
-}