Harden the content contracts and enforce the slug rule

Elternbeirat.Web/Features/Posts/PostDetail.razor.cs

@@ -15,7 +15,7 @@ public partial class PostDetail
     private static partial void LogLoadFailed(ILogger logger, string slug, Exception exception);
 
     [Parameter]
-    public string Slug { get; set; } = "";
+    public string Slug { get; set; } = string.Empty;
 
     [Inject]
     private PocketBaseClient PocketBase { get; set; } = default!;