Harden the content contracts and enforce the slug rule

Elternbeirat.Web.Tests/PocketBaseClientTests.cs

@@ -19,7 +19,6 @@ public sealed class PocketBaseClientTests(PocketBaseFixture pocketBase)
 
         // Two seeded events, earliest start first.
         events.Select(e => e.Title).ShouldBe(["Elternbeiratssitzung", "Herbstbasar"]);
-        events.ShouldAllBe(e => e.Public);
     }
 
     [Fact]