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

docs/demo-script.md

@@ -275,3 +275,26 @@ ACL → NRC → event-subscriber → projection → openbaar register shows INGE
 > The full round-trip — DigiD submit → public INGEDIEND → behandelaar goedkeurt in the werkbak →
 > public INGESCHREVEN — is the Playwright happy path (`tests/e2e/registration.spec.ts`), which now
 > drives the behandel portal in place of the old admin endpoint.
+
+## S-11 — Withdrawal: "trek aanvraag in" (#12, ADR-0014)
+
+A zorgprofessional can withdraw their own still-open registration from the self-service portal. The
+withdrawal is owner-scoped (the BFF forwards the DigiD token's bsn; the domain only lets the owner
+withdraw) and cancels the running workflow via a BPMN message event, so the case leaves the
+behandelaar's werkbak.
+
+```text
+# 1. Log in and submit at the self-service portal (http://localhost:8140/, jan-burger / test123),
+#    note the "Referentie" on the confirmation.
+# 2. Click "Trek aanvraag in" → the page confirms the registration is ingetrokken.
+# 3. In the behandel werkbak (http://localhost:8142/, merel-behandelaar) the registration no longer
+#    appears — its Beoordelen task was cancelled.
+```
+
+**The path:** self-service → BFF `POST /self-service/registrations/{id}/withdraw` (DigiD, owner-scoped)
+→ domain sets INGETROKKEN + correlates the `RegistratieIngetrokken` message to the process → the
+interrupting boundary event ends it → the werkbak drops the case.
+
+> DigiD submit → trek aanvraag in → ingetrokken is the Playwright happy path
+> (`tests/e2e/withdrawal.spec.ts`); the owner-scoping + workflow cancellation are covered by the
+> `Een registratie intrekken` acceptance scenarios and the domain live check.