fix: rev db model, added missing entities and helper functions

WebApp/Entities/MessageActivity.cs

@@ -0,0 +1,9 @@
+namespace WebApp.Entities
+{
+    public class MessageActivity
+    {
+        public int Recipient {  get; set; }
+        public int Sender { get; set; }
+        public DateTime RecipientLastActive { get; set; }
+    }
+}