Setup basic code for login.

API/Entities/AppUser.cs

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+using System;
+using System.ComponentModel.DataAnnotations;
+using API.Entities.Interfaces;
+
+
+namespace API.Entities
+{
+    public class AppUser : IHasConcurrencyToken
+    {
+        public int Id { get; set; }
+        public string UserName { get; set; }
+        public byte[] PasswordHash { get; set; }
+        public byte[] PasswordSalt { get; set; }
+        public DateTime Created { get; set; } = DateTime.Now;
+        public DateTime LastActive { get; set; }
+
+        [ConcurrencyCheck]
+        public uint RowVersion { get; set; }
+
+        public void OnSavingChanges()
+        {
+            RowVersion++;
+        }
+        
+        
+        
+    }
+}

API/Entities/Interfaces/IHasConcurrencyToken.cs

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+namespace API.Entities.Interfaces
+{
+    /// <summary>
+    /// An interface abstracting an entity that has a concurrency token.
+    /// </summary>
+    public interface IHasConcurrencyToken
+    {
+        /// <summary>
+        /// Gets the version of this row. Acts as a concurrency token.
+        /// </summary>
+        uint RowVersion { get; }
+
+        /// <summary>
+        /// Called when saving changes to this entity.
+        /// </summary>
+        void OnSavingChanges();
+
+    }
+}