Convert autoproxy timeout to use seconds

PluralKit.Core/Models/PKSystem.cs

@@ -18,7 +18,7 @@ namespace PluralKit.Core {
         public Instant Created { get; }
         public string UiTz { get; set; }
         public bool PingsEnabled { get; }
-        public int LatchTimeout { get; }
+        public int? LatchTimeout { get; }
 	    public PrivacyLevel DescriptionPrivacy { get; }
         public PrivacyLevel MemberListPrivacy { get;}
         public PrivacyLevel FrontPrivacy { get; }