Make Id property in Endpoint class non-nullable

The `Id` property in the `Endpoint` class was changed from a
nullable `long?` to a non-nullable `long`. This ensures that
the `Id` property is always assigned a value and cannot be
`null`, improving data integrity and aligning with potential
database or application requirements.

src/ReC.Domain/Entities/Endpoint.cs

@@ -10,7 +10,7 @@ public class Endpoint
     [Key]
     [Column("GUID")]
     [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
-    public long? Id { get; set; }
+    public long Id { get; set; }
 
     [Column("ACTIVE")]
     public bool? Active { get; set; }