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

infra/run-domain-check.sh

@@ -142,8 +142,10 @@ done
 [ -n "$task_id2" ] || { echo "FAIL — no Beoordelen task appeared for registration $reg_id2" >&2; docker logs "$dom" 2>&1 | tail -15 >&2; exit 1; }
 echo ">> Beoordelen task $task_id2 is waiting; withdrawing the registration via the domain"
 
+# Owner-scoped: the withdraw carries the same bsn the registration was submitted with (S-11c).
 docker run --rm --network "$net" curlimages/curl:latest \
-  -fsS -X POST "http://$dom_ip:8080/registrations/$reg_id2/withdraw" >/dev/null
+  -fsS -X POST "http://$dom_ip:8080/registrations/$reg_id2/withdraw" \
+  -H 'Content-Type: application/json' -d '{"bsn":"123456782"}' >/dev/null
 
 echo ">> asserting the process was cancelled (no Beoordelen task remains for the registration)"
 gone=""