add offline messaging feature

Data/Models/AuditFields.cs

@@ -1,9 +1,10 @@
 using System;
 using System.ComponentModel.DataAnnotations;
+using Data.Abstractions;
 
 namespace Stats.Models
 {
-    public class AuditFields
+    public class AuditFields : IAuditFields
     {
         [Required]
         public DateTime CreatedDateTime { get; set; } = DateTime.UtcNow;

Data/Models/Misc/EFInboxMessage.cs

@@ -0,0 +1,35 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using Data.Models.Client;
+using Data.Models.Server;
+using Stats.Models;
+
+namespace Data.Models.Misc
+{
+    public class EFInboxMessage : AuditFields
+    {
+        [Key]
+        public int InboxMessageId { get; set; }
+        
+        [Required]
+        public int SourceClientId { get; set; }
+        
+        [ForeignKey(nameof(SourceClientId))]
+        public EFClient SourceClient { get; set; }
+        
+        [Required]
+        public int DestinationClientId { get; set; }
+        
+        [ForeignKey(nameof(DestinationClientId))]
+        public EFClient DestinationClient { get; set; }
+        
+        public long? ServerId { get; set; }
+        
+        [ForeignKey(nameof(ServerId))]
+        public EFServer Server { get; set; }
+
+        public string Message { get; set; }
+        
+        public bool IsDelivered { get; set; }
+    }
+}