Add news post type with list and detail routes

Elternbeirat.Web/Services/Post.cs

@@ -0,0 +1,33 @@
+namespace Elternbeirat.Web.Services;
+
+/// <summary>
+/// A news post ("Beitrag"), read from a Markdown file in
+/// <c>Content/beitraege/</c>. Unlike a <see cref="Page"/>, a post has a date and
+/// is shown in a chronological list. The file name (without extension) is the
+/// slug.
+/// </summary>
+public sealed class Post
+{
+    /// <summary>
+    /// URL identifier of the post, taken from the file name without umlauts.
+    /// It appears in the route <c>/beitraege/{slug}</c>.
+    /// </summary>
+    public required string Slug { get; init; }
+
+    /// <summary>
+    /// Display title from the YAML front matter (<c>titel:</c>). Falls back to
+    /// the slug when no title is set.
+    /// </summary>
+    public required string Title { get; init; }
+
+    /// <summary>
+    /// Publication date from the front matter (<c>datum:</c>). Used to sort the
+    /// list newest first.
+    /// </summary>
+    public required DateOnly Date { get; init; }
+
+    /// <summary>
+    /// The post body rendered from Markdown to HTML (front matter excluded).
+    /// </summary>
+    public required string ContentHtml { get; init; }
+}