Harden the content contracts and enforce the slug rule

Elternbeirat.Contracts/Post.cs

@@ -13,8 +13,13 @@ public record Post
     /// <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 publication date as a calendar day.
@@ -34,28 +39,32 @@ public record Post
     /// </summary>
     /// <remarks>
     ///     Free text; may contain umlauts and spaces. For the URL, see <see cref="Slug"/>.
+    ///     <see langword="required"/>: a post without a heading 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 post body as Markdown.
     /// </summary>
     [JsonPropertyName("body")]
-    public string Body { get; init; } = "";
+    public string Body { get; init; } = string.Empty;
 
     /// <summary>
     ///     Gets the URL slug of the post, e.g. <c>"herbstbasar"</c> for <c>/posts/herbstbasar</c>.
     /// </summary>
     /// <value>
-    ///     A lowercase path segment without umlauts.
+    ///     A URL path segment: lowercase letters, digits and single hyphens as
+    ///     separators, no umlauts. PocketBase enforces this on save via a field
+    ///     pattern (<c>^[a-z0-9]+(-[a-z0-9]+)*$</c>), so every stored slug is already
+    ///     canonical and the app can use it verbatim in comparisons and generated URLs.
     /// </value>
+    /// <remarks>
+    ///     <see langword="required"/>: the slug is how a post is addressed, so a record
+    ///     without one is broken. It is a required field in PocketBase, so deserializing
+    ///     one that lacks it should fail loudly rather than yield a post with no URL.
+    /// </remarks>
     [JsonPropertyName("slug")]
-    public string Slug { get; init; } = "";
-
-    /// <summary>
-    ///     Gets a value indicating whether the post is visible to visitors of the site.
-    /// </summary>
-    [JsonPropertyName("public")]
-    public bool Public { get; init; }
+    public required string Slug { get; init; }
 }