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- **S-11b** — cancel the running Flowable process via a BPMN message event, so a withdrawn case leaves the behandelaar's werkbak.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.Api/Program.cs

@@ -26,6 +26,7 @@ builder.Services.AddHttpClient<IAclClient, AclHttpClient>();
 builder.Services.AddScoped<SubmitRegistration>();
 builder.Services.AddScoped<ApproveRegistration>();
 builder.Services.AddScoped<BeoordeelRegistratie>();
+builder.Services.AddScoped<WithdrawRegistration>();
 builder.Services.AddScoped<Werkbak>();
 builder.Services.AddScoped<OpenZaakWorker>();
 builder.Services.AddScoped<OpenZaakJobProcessor>();
@@ -74,6 +75,19 @@ app.MapPost("/registrations/{id}/decide", async (string id, DecideRequest body,
     return Results.NoContent();
 });
 
+// Withdraw a registration (S-11): the zorgprofessional pulls their own still-open submission back,
+// advancing it to INGETROKKEN. Idempotent. The BFF reaches this behind a digid token, owner-scoped
+// to the caller's bsn (S-11c); the domain trusts its callers (§8.3). Cancelling the running Flowable
+// process is a later sub-slice (S-11b).
+app.MapPost("/registrations/{id}/withdraw", async (string id, WithdrawRegistration withdraw, CancellationToken ct) =>
+{
+    if (!Guid.TryParse(id, out var guid))
+        return Results.NotFound();
+
+    await withdraw.HandleAsync(new WithdrawRegistrationCommand(new RegistrationId(guid)), ct);
+    return Results.NoContent();
+});
+
 // The behandelaar's werkbak (S-12): the registrations awaiting beoordeling, read from the open
 // Beoordelen user tasks (§8.2) and enriched with bsn + status. The BFF proxies this behind
 // medewerker-realm + behandelaar-role authorization; the domain trusts its callers (§8.3).