Harden the content contracts and enforce the slug rule

Elternbeirat.Web.Tests/PocketBaseFixture.cs

@@ -335,6 +335,6 @@ public sealed class PocketBaseFixture : IAsyncLifetime
     private sealed record AuthResponse
     {
         [System.Text.Json.Serialization.JsonPropertyName("token")]
-        public string Token { get; init; } = "";
+        public string Token { get; init; } = string.Empty;
     }
 }