Add power database support

Power/Service/Models/PowerStatus.cs

@@ -1,10 +1,12 @@
 using JetBrains.Annotations;
+using System;
 
 namespace ChrisKaczor.HomeMonitor.Power.Service.Models
 {
     [PublicAPI]
     public class PowerStatus
     {
+        public DateTimeOffset Timestamp { get; set; } = DateTimeOffset.UtcNow;
         public long Generation { get; set; }
         public long Consumption { get; set; }
     }