Refactor ContractType property in EnvelopeType.cs

Added a using directive for `EnvelopeGenerator.Domain.Constants` to utilize constants from that namespace. Changed the `ContractType` property from a nullable integer (`int?`) to a nullable `ContractType` enum or class, enhancing type safety and clarity.

EnvelopeGenerator.Domain/Entities/EnvelopeType.cs

@@ -1,5 +1,7 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
+using EnvelopeGenerator.Domain.Constants;
+
 #if NETFRAMEWORK
 using System;
 #endif
@@ -56,7 +58,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public int? ReminderIntervalDays { get; set; }
 
         [Column("CONTRACT_TYPE")]
-        public int? ContractType { get; set; }
+        public ContractType? ContractType { get; set; }
 
         [NotMapped]
         public bool TFA_Enabled { get; set; } = false;