Harden the content contracts and enforce the slug rule

Elternbeirat.Web.Tests/NavBuilderTests.cs

@@ -8,8 +8,10 @@ namespace Elternbeirat.Web.Tests;
 /// </summary>
 public class NavBuilderTests
 {
+    // Id is required on the record but irrelevant to navigation, so a fixed
+    // placeholder keeps these cases focused on slug/location/order/title.
     private static Page Page(string slug, string location, double order, string title) =>
-        new() { Slug = slug, Location = location, Order = order, Title = title };
+        new() { Id = "test", Slug = slug, Location = location, Order = order, Title = title };
 
     [Fact]
     public void Header_keeps_only_header_pages_ordered_by_order()