Fixed endpoints for /users and /messages.

code/MessengerBroker.Db/BrokerDbContext.cs

@@ -5,10 +5,10 @@ namespace MessengerBroker.Db
 {
     public class BrokerDbContext : DbContext
     {
-        public DbSet<Sync> Syncs { get; }
+        public DbSet<Sync> Syncs { get; set; }
 
-        public DbSet<User> Users { get; }
+        public DbSet<User> Users { get; set; }
 
-        public DbSet<Message> Messages { get; }
+        public DbSet<Message> Messages { get; set; }
     }
 }