Move date converters onto the contracts

CLAUDE.md

@@ -83,6 +83,12 @@ Inhalt liegt in PocketBase (`pb_data`), das separat gesichert wird (siehe
 - Datenzugriff über den typisierten `PocketBaseClient` (registriert via
   `AddHttpClient`), der pro Request liest — kein Start-Cache, kein Singleton mit
   Inhalten. Komponenten liegen feature-basiert unter `Features/<Bereich>/`.
+- JSON-Konverter liegen in `Contracts` und werden per `[JsonConverter]` **am Feld**
+  zugeordnet, nicht global auf den `JsonSerializerOptions` im Client registriert. So
+  ist am Contract sichtbar, wie ein Feld behandelt wird (z. B. `Event.Start` als
+  Zeitpunkt, `Post.Date` als Kalendertag), und `Contracts` bekommt keine Referenz
+  auf `PocketBase` (Zirkel). Datumsfelder: `DateTime`/`DateTime?` = Zeitpunkt
+  (Europe/Berlin), `DateOnly` = Kalendertag.
 - **Code durchgängig auf Englisch** — Typen, Member, Variablen, Kommentare,
   Skripte und Doku. Das gilt **auch für Domänenbegriffe**: im Code `Event`, nicht
   `Termin`; `Post`, nicht `Beitrag`. Deutsch bleibt ausschließlich, was ein

Elternbeirat.Contracts/DateOnlyConverter.cs

@@ -1,11 +1,11 @@
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
-namespace Elternbeirat.PocketBase;
+namespace Elternbeirat.Contracts;
 
 /// <summary>
 ///     JSON converter that reads and writes a PocketBase date as a calendar date
-///     (<see cref="DateOnly"/>), ignoring the time-of-day and the time zone.
+///     (<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
@@ -16,11 +16,14 @@ namespace Elternbeirat.PocketBase;
 ///     <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>
-/// <remarks>
-///     Registered on the client's <see cref="System.Text.Json.JsonSerializerOptions"/>
-///     and matched by the <see cref="DateOnly"/> field type, so a contract needs no
-///     per-field attribute (which would couple the contract to this project).
-/// </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/>

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>

Elternbeirat.Contracts/LocalDateTimeConverter.cs

@@ -2,7 +2,7 @@ using System.Globalization;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
-namespace Elternbeirat.PocketBase;
+namespace Elternbeirat.Contracts;
 
 /// <summary>
 ///     Converts between PocketBase date strings and Europe/Berlin wall-clock time.
@@ -71,6 +71,7 @@ internal static class WallClock
 ///     For optional dates, use <see cref="NullableLocalDateTimeConverter"/> instead.
 /// </remarks>
 /// <example>
+///     Apply it to a required date field of a contract:
 /// <code>
 ///     [JsonConverter(typeof(LocalDateTimeConverter))]
 ///     public DateTime Start { get; init; }
@@ -103,6 +104,7 @@ public sealed class LocalDateTimeConverter : JsonConverter<DateTime>
 ///     without an end). This value, like JSON <c>null</c>, is read as <see langword="null"/>.
 /// </remarks>
 /// <example>
+///     Apply it to an optional date field of a contract:
 /// <code>
 ///     [JsonConverter(typeof(NullableLocalDateTimeConverter))]
 ///     public DateTime? End { get; init; }

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>

Elternbeirat.PocketBase/PocketBaseClient.cs

@@ -24,7 +24,7 @@ namespace Elternbeirat.PocketBase;
 public sealed class PocketBaseClient(HttpClient http)
 {
     /// <summary>
-    ///     Serializer options that map PocketBase UTC dates to Berlin wall-clock time.
+    ///     Shared serializer options for every request; see <see cref="CreateJsonOptions"/>.
     /// </summary>
     private static readonly JsonSerializerOptions JsonOptions = CreateJsonOptions();
 
@@ -83,23 +83,16 @@ public sealed class PocketBaseClient(HttpClient http)
         => GetRecordsAsync<Faq>("faqs", "topic", ct);
 
     /// <summary>
-    ///     Creates the serializer options with the date converters registered: the
-    ///     wall-clock ones for points in time and the calendar-day one for plain dates.
+    ///     Creates the serializer options. Date handling lives on the contracts
+    ///     themselves via <c>[JsonConverter]</c> on each date field (e.g.
+    ///     <see cref="Contracts.Event.Start"/>, <see cref="Contracts.Post.Date"/>), so
+    ///     nothing date-related is registered here.
     /// </summary>
     /// <returns>
     ///     The configured options.
     /// </returns>
-    private static JsonSerializerOptions CreateJsonOptions()
-    {
-        var options = new JsonSerializerOptions
-        {
-            PropertyNameCaseInsensitive = true,
-        };
-        options.Converters.Add(new LocalDateTimeConverter());
-        options.Converters.Add(new NullableLocalDateTimeConverter());
-        options.Converters.Add(new DateOnlyConverter());
-        return options;
-    }
+    private static JsonSerializerOptions CreateJsonOptions() =>
+        new() { PropertyNameCaseInsensitive = true };
 
     /// <summary>
     ///     Gets all public records of a collection in a single request.