Move ReadOnly property from Envelope to EnvelopeDto

The ReadOnly property logic was shifted from the Envelope entity
to the EnvelopeDto record, ensuring that read-only status is
determined at the DTO layer rather than in the data model.

EnvelopeGenerator.Domain/Entities/Envelope.cs

@@ -108,10 +108,6 @@ public class Envelope
     [Column("ENVELOPE_TYPE")]
     public int? EnvelopeTypeId { get; set; }
 
-    [JsonIgnore]
-    [NotMapped]
-    public bool ReadOnly => EnvelopeTypeId == 2;
-
     [Column("CERTIFICATION_TYPE")]
     public int? CertificationType { get; set; }