feat(domain): withdrawal cancels the registratie process (S-11b, refs #12) (#89)

## What & why

Second sub-slice of **S-11 · Withdrawal (Flow 3)** (#12). S-11a (#88) made a withdrawal advance the aggregate to INGETROKKEN; this sub-slice **cancels the running Flowable process** so the withdrawn case leaves the behandelaar's werkbak.

- **BPMN** (`registratie.bpmn`): an interrupting message boundary event (`RegistratieIngetrokken`) on the `Beoordelen` task, routing to a dedicated "Registratie ingetrokken" end event.
- **Workflow Client**: `WithdrawBeoordelingAsync(executionId)` delivers `messageEventReceived` to the task's execution (PUT); `BeoordelingTask` now carries its `executionId`.
- **`WithdrawRegistration` handler**: after the domain transition, finds the open `Beoordelen` task for the registration and delivers the withdrawal message — best-effort, mirroring how the beoordeling completes its task.
- **Werkbak**: also filters out registrations that are no longer open, so a withdrawn case never surfaces even in the brief window before cancellation lands.
- **ADR-0014** records the decision (message event in BPMN vs. deleting the instance from code).
- **verify (`run-domain-check.sh`)**: a second registration parks at `Beoordelen`, is withdrawn via the domain, and the check asserts its `Beoordelen` task disappears — so verify-stack validates the live Flowable message correlation.

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

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing tests committed before the implementation (red → green per commit).
- [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 (BPMN redeploys on a fresh CI DB via flowable-init).
- [x] ADR added (ADR-0014).
- [x] Docs — the user-visible demo note lands with S-11c.

## Notes for reviewers

- Verified locally: `Big.Tests` 94/94 pass; `Big.Api` builds; `registratie.bpmn` is well-formed.
- The Flowable message-correlation REST shape is validated **live** by verify-stack (the Workflow Client unit tests stub the exchange and assert only the request shape, per ADR-0009) — the new `run-domain-check.sh` withdrawal step is that live check.
- Known gap (ADR-0014): a withdrawal that races ahead of the process reaching `Beoordelen` finds no task to cancel; the aggregate is still INGETROKKEN and the werkbak filter hides it, but that instance parks unattended. A process-level event subprocess would close the gap — deferred.

Reviewed-on: #89

docs/architecture/adr-0014-withdrawal-cancels-the-process.md

@@ -0,0 +1,72 @@
+# ADR-0014: Withdrawal cancels the registratie process via a BPMN message event
+
+- **Status:** Accepted
+- **Date:** 2026-07-16
+- **Deciders:** Respellion engineering
+- **Relates to:** S-11 (#12); builds on ADR-0009 (external-task worker / Workflow Client), ADR-0013
+  (behandel-portal wiring, the Beoordelen user task)
+
+## Context
+
+S-11 lets a zorgprofessional withdraw a still-open registration ("trek aanvraag in"). S-11a already
+advances the aggregate to INGETROKKEN (domain state). But the registratie process is still running in
+Flowable — parked at the `Beoordelen` user task — so without a second step the withdrawn registration
+would linger as work for a behandelaar. The withdrawal must also **cancel the running process**.
+
+Two questions shape this sub-slice.
+
+1. **How does the case get cancelled — in code, or in the BPMN model?**
+2. **How does a withdrawal correlate to the right running process instance?**
+
+## Decision
+
+**The BPMN models the cancellation as an interrupting message boundary event on the `Beoordelen`
+task; the Workflow Client correlates a `RegistratieIngetrokken` message to the task's execution.**
+
+- **Modelled in BPMN, not deleted from code.** The `Beoordelen` user task carries an interrupting
+  message boundary event (`RegistratieIngetrokken`) that routes to a dedicated "Registratie
+  ingetrokken" end event. The process's own model says *how* a withdrawal ends it — the Workflow
+  Client only delivers the message; it never reaches into Flowable to delete an instance. This keeps
+  the workflow's control flow in the workflow (§8.2) and leaves an audit trail in Flowable history
+  (the process ended via the ingetrokken path, not a raw delete).
+- **Correlated by the registration's own process instance.** The aggregate records its Flowable
+  process instance id at submit, so the `WithdrawRegistration` handler correlates directly by that
+  id — no task lookup. The Workflow Client asks Flowable for the execution **subscribed to** the
+  `RegistratieIngetrokken` message in that instance and delivers `messageEventReceived` to it.
+  Targeting the subscribed execution (not the user task's execution — a message boundary event's
+  subscription lives on its own execution) is what makes the correlation land.
+- **Best-effort, mirroring the beoordeling.** If no open `Beoordelen` task is found (the process has
+  not yet parked there — the `OpenZaakAanmaken` window — or has already ended), the withdrawal still
+  stands: the aggregate is INGETROKKEN and the werkbak filters it out regardless (S-11b). We complete
+  the domain transition first and cancel the workflow best-effort, exactly as `BeoordeelRegistratie`
+  completes its task best-effort.
+
+## Consequences
+
+**Positive**
+
+- The cancellation path is visible in `registratie.bpmn`; the Workflow Client stays the only code
+  that talks to Flowable and does not delete instances behind the model's back.
+- Reuses the existing task-query correlation — no new plumbing, no correlation store.
+- A withdrawn case leaves the werkbak (its `Beoordelen` task is cancelled), and the werkbak also
+  filters non-open registrations as a belt-and-braces for the brief window before cancellation lands.
+
+**Negative / costs**
+
+- A withdrawal raced ahead of the process reaching `Beoordelen` (during `OpenZaakAanmaken`, seconds)
+  finds no task to cancel, so that process instance runs on to `Beoordelen` and parks there with no
+  one to act on it (it is hidden from the werkbak by the status filter). Acceptable for this
+  reference at these volumes; a process-level interrupting event subprocess would close the gap and
+  is an additive follow-up if it matters.
+- The Flowable message-correlation REST shape is validated live (verify-stack), not in the
+  Workflow Client's unit tests, which stub the HTTP exchange and assert only the request shape
+  (consistent with ADR-0009).
+
+## Alternatives considered
+
+- **Delete the process instance from the Workflow Client** (`DELETE /runtime/process-instances/{id}`)
+  — rejected: it cancels the case but hides the reason from the BPMN model; the "why" lives in code,
+  not the process. The message event keeps the cancellation a first-class part of the workflow.
+- **Interrupting message event subprocess at process level** — more robust (correlates anytime,
+  closing the `OpenZaakAanmaken`-race gap), but a heavier BPMN construct; deferred as an additive
+  change if the race proves to matter.