Make "Sometimes" setting depend on season end date, rather than chance

osu.Game/Online/API/Requests/Responses/APISeasonalBackgrounds.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System;
 using System.Collections.Generic;
 using Newtonsoft.Json;
 
@@ -8,6 +9,9 @@ namespace osu.Game.Online.API.Requests.Responses
 {
     public class APISeasonalBackgrounds
     {
+        [JsonProperty("ends_at")]
+        public DateTimeOffset EndDate;
+
         [JsonProperty("backgrounds")]
         public List<APISeasonalBackground> Backgrounds { get; set; }
     }