Harden the content contracts and enforce the slug rule

Elternbeirat.Contracts/Event.cs

@@ -14,8 +14,13 @@ public record Event
     /// <summary>
     ///     Gets the PocketBase record id.
     /// </summary>
+    /// <remarks>
+    ///     <see langword="required"/>: every stored record has an id. Modeling it as
+    ///     required states which fields a record must carry, independent of the store,
+    ///     so a future data source has to supply them too.
+    /// </remarks>
     [JsonPropertyName("id")]
-    public string Id { get; init; } = "";
+    public required string Id { get; init; }
 
     /// <summary>
     ///     Gets the start of the event as Europe/Berlin wall-clock time.
@@ -24,10 +29,12 @@ public record Event
     ///     PocketBase stores the value in UTC; it is converted to Berlin local time on
     ///     deserialization, with daylight saving applied. An all-day event uses 00:00
     ///     as the time.
+    ///     <see langword="required"/>: an event without a start has no place on the
+    ///     calendar, and the field is required in PocketBase.
     /// </remarks>
     [JsonPropertyName("start")]
     [JsonConverter(typeof(LocalDateTimeConverter))]
-    public DateTime Start { get; init; }
+    public required DateTime Start { get; init; }
 
     /// <summary>
     ///     Gets the optional end of the event as Europe/Berlin wall-clock time.
@@ -42,8 +49,12 @@ public record Event
     /// <summary>
     ///     Gets the event name, e.g. <c>"Elternbeiratssitzung"</c>.
     /// </summary>
+    /// <remarks>
+    ///     <see langword="required"/>: an event without a name is incomplete, and the
+    ///     field is required in PocketBase.
+    /// </remarks>
     [JsonPropertyName("title")]
-    public string Title { get; init; } = "";
+    public required string Title { get; init; }
 
     /// <summary>
     ///     Gets the optional location, e.g. <c>"Aula"</c>.
@@ -52,7 +63,7 @@ public record Event
     ///     The location, or <see cref="string.Empty"/> if none is set.
     /// </value>
     [JsonPropertyName("location")]
-    public string Location { get; init; } = "";
+    public string Location { get; init; } = string.Empty;
 
     /// <summary>
     ///     Gets the optional note, e.g. <c>"Anmeldung erforderlich"</c>.
@@ -61,13 +72,7 @@ public record Event
     ///     The note, or <see cref="string.Empty"/> if none is set.
     /// </value>
     [JsonPropertyName("note")]
-    public string Note { get; init; } = "";
-
-    /// <summary>
-    ///     Gets a value indicating whether the event is visible to visitors of the site.
-    /// </summary>
-    [JsonPropertyName("public")]
-    public bool Public { get; init; }
+    public string Note { get; init; } = string.Empty;
 
     /// <summary>
     ///     Gets a value indicating whether the event has a wall-clock time, as opposed