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/EventSubscriber.Application.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+</Project>

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

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+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}";
+}

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

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+namespace EventSubscriber.Application;
+
+/// <summary>
+/// Projects inbound NRC notifications into the read projection. Tolerates duplicate and
+/// out-of-order deliveries (CLAUDE.md §8.6): the notification log dedups, and the projection
+/// upsert is idempotent on the zaak id. Rebuilds the projection by replaying the log.
+/// </summary>
+public sealed class NotificationProjector(INotificationLog log, IProjectionStore store)
+{
+    /// <summary>Handle one inbound notification. Ignores anything that is not a zaak-created event.</summary>
+    public Task HandleAsync(Notification notification, CancellationToken ct = default)
+        // RED: not yet implemented — the smallest change to make the tests pass comes next.
+        => Task.CompletedTask;
+
+    /// <summary>Rebuild the projection from the durable notification log (PRD §8.4).</summary>
+    public Task RebuildAsync(CancellationToken ct = default)
+        // RED: not yet implemented.
+        => Task.CompletedTask;
+}

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

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+namespace EventSubscriber.Application;
+
+/// <summary>
+/// The durable log of notifications the subscriber has accepted. It is both the idempotency
+/// guard (a replayed notification is recognised and dropped) and the rebuild source: the
+/// projection is a derived artefact (PRD §8.4) regenerated by replaying this log, so a rebuild
+/// needs no access to OpenZaak (CLAUDE.md §8.1). Implemented in Infrastructure over Postgres.
+/// </summary>
+public interface INotificationLog
+{
+    /// <summary>
+    /// Record a notification. Returns <c>true</c> if it was newly recorded, <c>false</c> if an
+    /// entry with the same key already existed (a duplicate delivery). Must be atomic so that
+    /// concurrent duplicate deliveries cannot both observe <c>true</c>.
+    /// </summary>
+    Task<bool> TryRecordAsync(RecordedNotification notification, CancellationToken ct = default);
+
+    /// <summary>Every accepted notification, for rebuilding the projection.</summary>
+    Task<IReadOnlyList<RecordedNotification>> AllAsync(CancellationToken ct = default);
+}
+
+/// <summary>A notification that has been accepted, retaining what a rebuild needs to recompute its projection row.</summary>
+public sealed record RecordedNotification(string Key, string Actie, string ZaakId);
+
+/// <summary>The read projection store. Owned by the projection bounded context (ADR-0008); the
+/// subscriber writes to it and the projection-api reads it.</summary>
+public interface IProjectionStore
+{
+    /// <summary>Insert or update the row for <see cref="RegisterEntry.Id"/>. Idempotent on the id.</summary>
+    Task UpsertAsync(RegisterEntry entry, CancellationToken ct = default);
+
+    /// <summary>Remove every row — the first step of a rebuild.</summary>
+    Task ClearAsync(CancellationToken ct = default);
+
+    /// <summary>Every projection row (used by tests and the rebuild verification).</summary>
+    Task<IReadOnlyList<RegisterEntry>> AllAsync(CancellationToken ct = default);
+}

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

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+namespace EventSubscriber.Application;
+
+/// <summary>
+/// A row of the rebuildable read projection (PRD §8.4). For the minimal slice it carries
+/// the zaak id and a status; <c>Bsn</c> and <c>NaamPlaceholder</c> are part of the schema but
+/// are deferred — the NRC notification does not carry them and the subscriber may not read
+/// OpenZaak directly (CLAUDE.md §8.1). Populating them is a follow-up (ADR-0008).
+/// </summary>
+public sealed record RegisterEntry(
+    string Id,
+    string Status,
+    string? Bsn = null,
+    string? NaamPlaceholder = null);
+
+/// <summary>The projection statuses the walking skeleton knows about.</summary>
+public static class RegistrationStatus
+{
+    /// <summary>A zaak has been created; the registration is submitted.</summary>
+    public const string Ingediend = "INGEDIEND";
+}