Move date converters onto the contracts

Elternbeirat.Contracts/DateOnlyConverter.cs

@@ -0,0 +1,47 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Elternbeirat.Contracts;
+
+/// <summary>
+///     JSON converter that reads and writes a PocketBase date as a calendar date
+///     (<see cref="DateOnly"/>), keeping the day and dropping the time-of-day.
+/// </summary>
+/// <remarks>
+///     PocketBase has no date-only field type: it stores every date as a UTC
+///     instant with a time-of-day (e.g. an editor's <c>2 Jan 00:00</c> Berlin is
+///     stored as <c>"2026-01-01 22:00:00.000Z"</c>). The day the editor meant is
+///     therefore the day <em>in Berlin</em>, not the day of the raw UTC string. This
+///     converter reuses the same UTC-&gt;Berlin conversion as
+///     <see cref="LocalDateTimeConverter"/> and then keeps only the date, so the day
+///     shown is the day that was entered -- with no time-of-day and no off-by-one.
+/// </remarks>
+/// <example>
+///     Apply it to a calendar-day field of a contract:
+/// <code>
+///     [JsonConverter(typeof(DateOnlyConverter))]
+///     public DateOnly Date { get; init; }
+/// </code>
+/// </example>
+/// <seealso cref="WallClock"/>
+public sealed class DateOnlyConverter : JsonConverter<DateOnly>
+{
+    /// <inheritdoc/>
+    /// <exception cref="FormatException">
+    ///     The stored value is not a valid date string.
+    /// </exception>
+    public override DateOnly Read(
+        ref Utf8JsonReader reader,
+        Type typeToConvert,
+        JsonSerializerOptions options)
+        => WallClock.Parse(reader.GetString()) is { } berlin
+            ? DateOnly.FromDateTime(berlin)
+            : default;
+
+    /// <inheritdoc/>
+    public override void Write(
+        Utf8JsonWriter writer,
+        DateOnly value,
+        JsonSerializerOptions options)
+        => writer.WriteStringValue(WallClock.Format(value.ToDateTime(TimeOnly.MinValue)));
+}