test(event-subscriber): project zaak-created notifications into the read projection (refs #7)

Failing unit + acceptance tests for the Event Subscriber's NotificationProjector:
a zaken/zaak/create notification yields one INGEDIEND projection row, duplicate
deliveries collapse to one row, non-zaak/non-create notifications are ignored, and
a rebuild repopulates the projection from the durable notification log (PRD §8.4).

The projector is a no-op stub so the tests compile and fail on the assertions; the
implementation follows in the green commit. The notification log doubles as the
idempotency guard and rebuild source so a rebuild needs no OpenZaak access (§8.1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/event-subscriber/EventSubscriber.Application/Notification.cs

@@ -0,0 +1,30 @@
+namespace EventSubscriber.Application;
+
+/// <summary>
+/// An inbound NRC (Open Notificaties) notification, as Open Notificaties POSTs it to an
+/// abonnement callback. Only the fields the projection needs are modelled; the full ZGW
+/// "Notificatie" resource also carries <c>aanmaakdatum</c> and <c>kenmerken</c> which the
+/// minimal projection ignores (bsn is deferred — see ADR-0008). For a <c>zaken</c>/<c>zaak</c>/<c>create</c>
+/// notification <c>hoofdObject</c> and <c>resourceUrl</c> are both the created zaak's URL.
+/// </summary>
+public sealed record Notification(
+    string Kanaal,
+    string Resource,
+    string Actie,
+    Uri ResourceUrl)
+{
+    /// <summary>The only event the walking-skeleton projection reacts to: a zaak being created.</summary>
+    public bool IsZaakCreated =>
+        Kanaal == "zaken" && Resource == "zaak" && Actie == "create";
+
+    /// <summary>The zaak UUID — the trailing path segment of the resource URL — used as the projection key.</summary>
+    public string ZaakId => ResourceUrl.Segments[^1].Trim('/');
+
+    /// <summary>
+    /// A deterministic dedup key. Open Notificaties carries no notification id and may
+    /// redeliver, so the key is derived from the immutable notification content: two
+    /// deliveries of the same zaak-create collapse to one. (NRC may also deliver
+    /// out of order; the projector tolerates that — order does not change the outcome.)
+    /// </summary>
+    public string IdempotencyKey => $"{Kanaal}:{Resource}:{Actie}:{ResourceUrl}";
+}