S-09b: Approval flow — temp admin endpoint + status transition to projection (#77)

## What & why

S-09b (#75, split from #10) — the **approval flow** that completes the walking skeleton. A behandelaar can now approve a submitted registration; the entry flips from `INGEDIEND` to `INGESCHREVEN` in the public register. Flow: `POST /registrations/{id}/approve` (domain) → ACL sets the zaak eindstatus (ZGW `/statussen`) → OpenZaak → NRC → event-subscriber → projection → openbaar.

## Changes (bottom-up, each red→green TDD)

- **Domain** — `RegistrationStatus.Ingeschreven` + `Registration.Approve()` (guards: opened zaak, only from INGEDIEND); `ApproveRegistration` use case (idempotent) + temp `POST /registrations/{id}/approve` endpoint; `IAclClient.ApproveZaakAsync`.
- **ACL** — resolves the zaaktype's **eindstatus** from the catalogus (`isEindstatus` / highest volgnummer) and POSTs a ZGW status; exposed as `POST /statussen`. Unit + real-OpenZaak integration test.
- **Event-subscriber** — binds NRC `hoofdObject`, projects a `status`/`create` as `INGESCHREVEN` keyed on the zaak (updates the existing row), **without reading OpenZaak** (§8.1). Retains the ZGW `resource` in the log (new column + EF migration) so a rebuild reproduces the status.
- **e2e** — extended: submit → public INGEDIEND → approve → public INGESCHREVEN.
- **Docs** — ADR-0011 (the two non-obvious decisions + the walking-skeleton assumption) + demo note.

## Key decisions (see ADR-0011)

- **ACL discovers the eindstatus** (chosen over injecting a statustype URL): no new config/seed plumbing, domain stays ZGW-ignorant.
- **Any post-creation status-set ⇒ INGESCHREVEN**: in the walking skeleton the only status ever set after creation is the approval, and the subscriber may not read ZGW — documented to tighten when more transitions arrive (S-12+).

## Verification

- All .NET unit suites green locally (domain 47, acl 11, event-subscriber 14, bff 16, acceptance 7); Release build + `dotnet format` clean.
- No new compose config (the eindstatus-discovery approach avoided it).
- The real-OpenZaak integration test (ACL status-set) and the full submit→approve→visible e2e run in CI `verify-stack` (live NRC→projection + selectielijst egress, not reproducible locally).

closes #75

Reviewed-on: #77

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

@@ -11,14 +11,23 @@ public sealed record Notification(
     string Kanaal,
     string Resource,
     string Actie,
-    Uri ResourceUrl)
+    Uri ResourceUrl,
+    Uri? HoofdObject = null)
 {
-    /// <summary>The only event the walking-skeleton projection reacts to: a zaak being created.</summary>
+    /// <summary>A zaak being created — projected as INGEDIEND.</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 status being set on a zaak — the approval, projected as INGESCHREVEN (S-09b). In the
+    /// walking skeleton the only status ever set after creation is the approval, and the subscriber may
+    /// not read OpenZaak (§8.1), so any status-create is taken as the approval.</summary>
+    public bool IsZaakStatusSet =>
+        Kanaal == "zaken" && Resource == "status" && Actie == "create";
+
+    /// <summary>The zaak UUID used as the projection key. For a status notification the resource URL is
+    /// the status, so the zaak comes from <c>hoofdObject</c> (which, for a zaak-create, equals the
+    /// resource URL) — see the NRC payload note.</summary>
+    public string ZaakId => (HoofdObject ?? ResourceUrl).Segments[^1].Trim('/');
 
     /// <summary>
     /// A deterministic dedup key. Open Notificaties carries no notification id and may