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.Tests/ApproveRegistrationTests.cs

@@ -0,0 +1,89 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class ApproveRegistrationTests
+{
+    private static Registration WithZaak(string bsn = "123456782")
+    {
+        var registration = Registration.Submit(bsn);
+        registration.AttachZaak(FakeAclClient.DefaultZaakUrl);
+        return registration;
+    }
+
+    [Fact]
+    public async Task Approving_sets_the_zaak_status_via_the_acl_and_marks_the_registration_ingeschreven()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var registration = WithZaak();
+        store.Seed(registration);
+        var handler = new ApproveRegistration(store, acl);
+
+        await handler.HandleAsync(new ApproveRegistrationCommand(registration.Id));
+
+        var saved = await store.GetAsync(registration.Id);
+        Assert.Equal(RegistrationStatus.Ingeschreven, saved!.Status);
+        Assert.Equal(FakeAclClient.DefaultZaakUrl, acl.ApprovedZaakUrl);
+        Assert.Equal(1, acl.ApproveCallCount);
+        // The approved aggregate is persisted (not just mutated in memory).
+        Assert.Equal(1, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Rejects_a_null_command_without_touching_the_store_or_acl()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var handler = new ApproveRegistration(store, acl);
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => handler.HandleAsync(null!));
+        Assert.Equal(0, acl.ApproveCallCount);
+    }
+
+    [Fact]
+    public async Task Approving_an_unknown_registration_throws_and_does_not_call_the_acl()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var handler = new ApproveRegistration(store, acl);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => handler.HandleAsync(new ApproveRegistrationCommand(RegistrationId.New())));
+        Assert.Contains("No registration", ex.Message);
+        Assert.Equal(0, acl.ApproveCallCount);
+    }
+
+    [Fact]
+    public async Task Approving_before_the_zaak_is_opened_throws_without_calling_the_acl()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var registration = Registration.Submit("123456782"); // no zaak yet
+        store.Seed(registration);
+        var handler = new ApproveRegistration(store, acl);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => handler.HandleAsync(new ApproveRegistrationCommand(registration.Id)));
+        Assert.Contains("no zaak", ex.Message);
+        Assert.Equal(0, acl.ApproveCallCount);
+    }
+
+    [Fact]
+    public async Task Re_approving_an_already_ingeschreven_registration_is_idempotent()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var registration = WithZaak();
+        store.Seed(registration);
+        var handler = new ApproveRegistration(store, acl);
+
+        await handler.HandleAsync(new ApproveRegistrationCommand(registration.Id));
+        await handler.HandleAsync(new ApproveRegistrationCommand(registration.Id));
+
+        // The second approval is a no-op: the ACL is not asked to set the status again.
+        Assert.Equal(1, acl.ApproveCallCount);
+        Assert.Equal(RegistrationStatus.Ingeschreven, (await store.GetAsync(registration.Id))!.Status);
+    }
+}