refactor(event-subscriber): drop the hoofdObject fallback (refs #153)

For a `resource: object` notification Objecten sends the object as both hoofdObject and
resourceUrl — the object *is* the main resource — so `HoofdObject ?? ResourceUrl` was a
branch that can never take its left side and that no test could distinguish. It came across
from the zaken path, where hoofdObject genuinely differed (the zaak behind a status).

Tests unchanged and green.

services/event-subscriber/EventSubscriber.Api/Program.cs

@@ -84,11 +84,12 @@ app.MapPost("/admin/rebuild", async (NotificationProjector projector, Cancellati
 
 await app.RunAsync();
 
-/// <summary>The NRC notification body, as Open Notificaties POSTs it. Only the fields the
-/// projection needs are bound; <c>aanmaakdatum</c>/<c>kenmerken</c> are ignored for the minimal slice.</summary>
-public sealed record NotificationDto(string Kanaal, string Resource, string Actie, Uri ResourceUrl, Uri? HoofdObject = null)
+/// <summary>The NRC notification body, as Open Notificaties POSTs it. Only the fields the projector
+/// needs are bound; <c>aanmaakdatum</c>, <c>kenmerken</c> and <c>hoofdObject</c> are ignored — for a
+/// register write hoofdObject is the same object as resourceUrl (ADR-0030).</summary>
+public sealed record NotificationDto(string Kanaal, string Resource, string Actie, Uri ResourceUrl)
 {
-    public Notification ToNotification() => new(Kanaal, Resource, Actie, ResourceUrl, HoofdObject);
+    public Notification ToNotification() => new(Kanaal, Resource, Actie, ResourceUrl);
 }
 
 public partial class Program