Add LangCode property to EmailTemplate entity

Introduced a required LangCode property to the EmailTemplate entity, mapped to the LANG_CODE column in the database as varchar(5). This supports language-specific email templates.

EnvelopeGenerator.Domain/Entities/EmailTemplate.cs

@@ -24,6 +24,10 @@ namespace EnvelopeGenerator.Domain.Entities
         [Column("SUBJECT", TypeName = "nvarchar(512)")]
         public string Subject { get; set; }
 
+        [Required]
+        [Column("LANG_CODE", TypeName = "varchar(5)")]
+        public string LangCode { get; set; }
+
         [Required]
         [Column("ADDED_WHEN", TypeName = "datetime")]
         [DefaultValue("GETDATE()")]