Move date converters onto the contracts

Elternbeirat.Contracts/Post.cs

@@ -1,4 +1,3 @@
-using System.ComponentModel;
 using System.Text.Json.Serialization;
 
 namespace Elternbeirat.Contracts;
@@ -27,6 +26,7 @@ public record Post
     ///     time-of-day, no off-by-one at midnight. See <see cref="DateOnlyConverter"/>.
     /// </remarks>
     [JsonPropertyName("date")]
+    [JsonConverter(typeof(DateOnlyConverter))]
     public DateOnly Date { get; init; }
 
     /// <summary>