feat(bff): owner-scoped self-service withdraw endpoint (S-11c-1, refs #12) (#90)

## What & why

Third sub-slice of **S-11 · Withdrawal (Flow 3)** (#12) — the **owner-scoped BFF withdraw endpoint** (backend). S-11a/b made a withdrawal transition the aggregate and cancel the workflow; this adds the citizen-facing entry point through the BFF, gated to the registration's owner.

- **Domain**: `WithdrawRegistrationCommand` carries the caller's `bsn`; the handler returns a `WithdrawOutcome` and refuses a bsn that doesn't own the registration. Unknown and not-owned are **both 404** (indistinguishable — ownership isn't revealed). `POST /registrations/{id}/withdraw` takes `{bsn}` and maps the outcome (204/404).
- **BFF**: `POST /self-service/registrations/{id}/withdraw` (DigiD-authenticated) forwards the token's `bsn` to the domain and relays 204/404. The BFF authenticates; the domain owner-scopes (an aggregate invariant, not the domain doing auth).
- OpenAPI spec + Angular client regenerated for the new endpoint.
- `run-domain-check.sh` withdrawal step now sends the owner `bsn` (verify-stack).

Refs #12 — the self-service "trek aanvraag in" button + e2e (S-11c-2) closes it.

## 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] No ADR needed (owner-scoping is an aggregate invariant; no boundary change).
- [x] Docs — the user-visible demo note lands with S-11c-2.

## Notes for reviewers

- **Full local gate run before pushing this time** (lessons from #89): `dotnet format --verify-no-changes` clean; `make unit` green — Acl 27, EventSubscriber 19, BFF 30, Acceptance 9, Big 95; `api-client` lint+test green.
- Owner mismatch returns 404 (not 403) so the portal can't be used to probe which references exist.

Reviewed-on: #90

tests/acceptance/Support/BffAcceptanceHost.cs

@@ -69,6 +69,9 @@ public sealed class CapturingDomainClient : IDomainClient
         return Task.FromResult(new SubmitAccepted("reg-acc-1", "Ingediend"));
     }
 
+    public Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+        => Task.FromResult(true);
+
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<WerkbakItem>>([]);