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/domain/Big.Application/ApproveRegistration.cs

@@ -0,0 +1,36 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A behandelaar's decision to approve a registration, in domain language.</summary>
+public sealed record ApproveRegistrationCommand(RegistrationId RegistrationId);
+
+/// <summary>
+/// The approve use case (S-09b): set the registration's zaak to its final status via the ACL (§8.1),
+/// then advance the aggregate to INGESCHREVEN. Idempotent — a redelivered or repeated approval of an
+/// already-approved registration is a no-op, so the ACL is not asked to set the status twice. The
+/// zaak status is the projection's source of truth (it flows back over NRC); the aggregate transition
+/// keeps the domain's own view consistent.
+/// </summary>
+public sealed class ApproveRegistration(IRegistrationStore store, IAclClient acl)
+{
+    public async Task HandleAsync(ApproveRegistrationCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        var registration = await store.GetAsync(command.RegistrationId, ct)
+            ?? throw new InvalidOperationException($"No registration {command.RegistrationId} to approve.");
+
+        // A repeated approval is a no-op: don't set the zaak status a second time.
+        if (registration.Status == RegistrationStatus.Ingeschreven)
+            return;
+
+        if (registration.ZaakUrl is null)
+            throw new InvalidOperationException(
+                $"Registration {command.RegistrationId} has no zaak yet; it cannot be approved.");
+
+        await acl.ApproveZaakAsync(registration.ZaakUrl, ct);
+        registration.Approve();
+        await store.SaveAsync(registration, ct);
+    }
+}