Move date converters onto the contracts

Elternbeirat.Contracts/Event.cs

@@ -26,6 +26,7 @@ public record Event
     ///     as the time.
     /// </remarks>
     [JsonPropertyName("start")]
+    [JsonConverter(typeof(LocalDateTimeConverter))]
     public DateTime Start { get; init; }
 
     /// <summary>
@@ -35,6 +36,7 @@ public record Event
     ///     The end time, or <see langword="null"/> if the event has no end.
     /// </value>
     [JsonPropertyName("end")]
+    [JsonConverter(typeof(NullableLocalDateTimeConverter))]
     public DateTime? End { get; init; }
 
     /// <summary>