feat(domain): withdrawal — INGETROKKEN transition + endpoint (S-11a, refs #12) (#88)

## What & why

First sub-slice of **S-11 · Withdrawal (Flow 3)** (#12). A zorgprofessional can withdraw a still-open registration ("trek aanvraag in"); this sub-slice delivers the **domain transition + endpoint**, mirroring how S-12a shipped the beoordeling decision model on its own (#82).

- `RegistrationStatus.Ingetrokken` (terminal).
- `Registration.Withdraw()` — allowed from INGEDIEND or IN_BEHANDELING, needs no zaak, idempotent, and rejected once the registration has been decided (INGESCHREVEN/AFGEWEZEN).
- `WithdrawRegistration` application handler (load → withdraw → persist; repeated withdrawal is a no-op).
- `POST /registrations/{id}/withdraw` on the domain API.

Demoable: `POST /registrations/{id}/withdraw` → `GET /registrations/{id}` shows `INGETROKKEN`.

Refs #12 (not closing — see below).

## Scope / follow-ups

S-11 is bigger than one slice, so it is split (CLAUDE.md §13), like S-12 was:
- **S-11a (this PR)** — domain withdrawal transition + endpoint.
- **S-11b** — cancel the running Flowable process via a BPMN message event, so a withdrawn case leaves the behandelaar's werkbak.
- **S-11c** — owner-scoped BFF self-service withdraw endpoint + "trek aanvraag in" button + e2e.

Cancelling the Flowable process is deliberately deferred (documented in `WithdrawRegistration`), exactly as the beoordeling's rejection deferred its zaak propagation. #12 stays open until S-11c.

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing test committed before the implementation.
- [x] Implementation makes the test pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected (no infra/contract change).
- [x] Docs — none needed for this backend sub-slice; the user-visible demo note lands with S-11c.
- [x] No ADR needed — mirrors existing aggregate/handler/endpoint patterns; no boundary change.

## Notes for reviewers

- Verified locally: `Big.Tests` 89/89 pass; `Big.Api` builds clean.
- The domain trusts its callers (§8.3); owner-scoping by the caller's bsn is enforced at the BFF in S-11c.

Reviewed-on: #88

services/domain/Big.Application/WithdrawRegistration.cs

@@ -0,0 +1,32 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A zorgprofessional's request to withdraw their own registration ("trek aanvraag in").</summary>
+public sealed record WithdrawRegistrationCommand(RegistrationId RegistrationId);
+
+/// <summary>
+/// The withdrawal use case (S-11): a zorgprofessional pulls a still-open registration back. It
+/// advances the aggregate to INGETROKKEN and persists it. Idempotent — a repeated or redelivered
+/// withdrawal of an already-withdrawn registration is a no-op (the aggregate is not persisted again).
+/// Cancelling the running Flowable process (so the case leaves the behandelaar's werkbak) is a later
+/// sub-slice (S-11b, via a BPMN message event); this sub-slice owns the domain transition only —
+/// mirroring how the beoordeling's rejection deferred its zaak propagation.
+/// </summary>
+public sealed class WithdrawRegistration(IRegistrationStore store)
+{
+    public async Task HandleAsync(WithdrawRegistrationCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        var registration = await store.GetAsync(command.RegistrationId, ct)
+            ?? throw new InvalidOperationException($"No registration {command.RegistrationId} to withdraw.");
+
+        // A repeated withdrawal is a no-op: don't persist the already-withdrawn aggregate again.
+        if (registration.Status == RegistrationStatus.Ingetrokken)
+            return;
+
+        registration.Withdraw();
+        await store.SaveAsync(registration, ct);
+    }
+}