feat(self-service): trek aanvraag in — withdrawal action (S-11c-2, closes #12) (#93)

## What & why

Final sub-slice of **S-11 · Withdrawal (Flow 3)** — the user-facing "trek aanvraag in" action, which **closes #12**.

- **self-service portal**: the submit confirmation gains a **"Trek aanvraag in"** button. It withdraws the just-submitted registration via `postSelfServiceRegistrationsIdWithdraw(reference)`; success shows an *ingetrokken* confirmation, a failure is surfaced (`role="alert"`) and the action stays available — same confirm-and-surface pattern as submit.
- **acceptance**: `Een registratie intrekken` — owner withdraws → INGETROKKEN + workflow cancelled; a different bsn is reported not-found.
- **e2e**: `withdrawal.spec.ts` — DigiD submit → trek aanvraag in → the portal confirms ingetrokken.
- **docs**: demo-script + frontend-decisions.

Together with S-11a (#88), S-11b (#89), S-11c-1 (#90), this completes the flow: citizen withdraws → domain INGETROKKEN → BPMN message event cancels the process → the case leaves the behandelaar's werkbak.

Closes #12

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing tests committed before the implementation.
- [x] Implementation makes the tests pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected.
- [x] Docs updated (demo-script + frontend-decisions).
- [x] ADR — ADR-0014 (from S-11b) covers the cancellation decision; nothing new here.

## Notes for reviewers

- Full local gate run before pushing: `dotnet format --verify-no-changes` clean; `make unit` green (Acceptance **11** incl. the 2 new withdrawal scenarios, Big 95, BFF 30, Acl 27, EventSubscriber 19); self-service lint/test/build green (9 tests, incl. the 2 new withdraw tests).
- `withdrawal.spec.ts` waits on the *ingetrokken* confirmation (which only renders after the withdraw POST returns), so it can't cancel the request early (the 499 lesson from #87). Live-validated by verify-stack.

Reviewed-on: #93

tests/acceptance/Steps/EenRegistratieIntrekkenSteps.cs

@@ -0,0 +1,59 @@
+using Acceptance.Support;
+using Big.Application;
+using Big.Domain;
+using Reqnroll;
+using Xunit;
+
+namespace Acceptance.Steps;
+
+/// <summary>Bindings for <c>EenRegistratieIntrekken.feature</c> (S-11). Submits a registration (which
+/// records its process) and then applies the WithdrawRegistration use case against in-memory ports;
+/// one instance per scenario. Scoped to this feature so its "the registration has status" step does
+/// not clash with the identically phrased steps in the other features.</summary>
+[Binding]
+[Scope(Feature = "Een registratie intrekken")]
+public sealed class EenRegistratieIntrekkenSteps
+{
+    private const string OwnerBsn = "123456782";
+
+    private readonly InMemoryRegistrationStore _store = new();
+    private readonly InMemoryWorkflowClient _workflow = new();
+    private RegistrationId _id;
+    private WithdrawOutcome _outcome;
+
+    [Given("a submitted registration with a running process")]
+    public async Task GivenASubmittedRegistrationWithARunningProcess()
+        => _id = await new SubmitRegistration(_store, _workflow).HandleAsync(new SubmitRegistrationCommand(OwnerBsn));
+
+    [When("the zorgprofessional withdraws it")]
+    public async Task WhenTheZorgprofessionalWithdrawsIt()
+        => _outcome = await new WithdrawRegistration(_store, _workflow).HandleAsync(
+            new WithdrawRegistrationCommand(_id, OwnerBsn));
+
+    [When("a different zorgprofessional tries to withdraw it")]
+    public async Task WhenADifferentZorgprofessionalTriesToWithdrawIt()
+        => _outcome = await new WithdrawRegistration(_store, _workflow).HandleAsync(
+            new WithdrawRegistrationCommand(_id, "999999990"));
+
+    [Then("the withdrawal succeeds")]
+    public void ThenTheWithdrawalSucceeds() => Assert.Equal(WithdrawOutcome.Withdrawn, _outcome);
+
+    [Then("the withdrawal is reported not found")]
+    public void ThenTheWithdrawalIsReportedNotFound() => Assert.Equal(WithdrawOutcome.NotFound, _outcome);
+
+    [Then("the registration has status \"(.*)\"")]
+    public async Task ThenTheRegistrationHasStatus(string expected)
+    {
+        var registration = await _store.GetAsync(_id);
+        Assert.NotNull(registration);
+        Assert.Equal(expected, registration.Status.ToString().ToUpperInvariant());
+    }
+
+    [Then("the running process is cancelled")]
+    public void ThenTheRunningProcessIsCancelled()
+        => Assert.Equal(InMemoryWorkflowClient.StartedProcessInstanceId, _workflow.WithdrawnProcessInstanceId);
+
+    [Then("the running process is not cancelled")]
+    public void ThenTheRunningProcessIsNotCancelled()
+        => Assert.Null(_workflow.WithdrawnProcessInstanceId);
+}