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made a withdrawal advance the aggregate to INGETROKKEN; this sub-slice **cancels the running Flowable process** so the withdrawn case leaves the behandelaar's werkbak.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- **Werkbak**: also filters out registrations that are no longer open, so a withdrawn case never surfaces even in the brief window before cancellation lands.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- **verify (`run-domain-check.sh`)**: a second registration parks at `Beoordelen`, is withdrawn via the domain, and the check asserts its `Beoordelen` task disappears — so verify-stack validates the live Flowable message correlation.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

workflows/registratie.bpmn

@@ -11,7 +11,13 @@
        S-03 added the external task (Workflow Client / ACL). S-12 adds the behandelaar's beoordeling
        as a user task: the process parks here until a behandelaar claims and completes it with a
        `besluit` (goedkeuren/afwijzen). The registrationId set at start rides along as a process
-       variable so the werkbak can correlate each task back to its aggregate. -->
+       variable so the werkbak can correlate each task back to its aggregate.
+       S-11 adds withdrawal: while parked at Beoordelen the citizen can trek de aanvraag in — an
+       interrupting message boundary event (RegistratieIngetrokken) cancels the task and ends the
+       process via a dedicated "ingetrokken" end (ADR-0014). The Workflow Client delivers the message
+       to the task's execution; the BPMN owns the cancellation path. -->
+  <message id="Message_Ingetrokken" name="RegistratieIngetrokken"/>
+
   <process id="registratie" name="Registratie ontvangen" isExecutable="true">
 
     <startEvent id="start" name="Registratie ontvangen"/>
@@ -29,6 +35,16 @@
     <sequenceFlow id="flow3" sourceRef="Beoordelen" targetRef="end"/>
 
     <endEvent id="end" name="Registratie beoordeeld"/>
+
+    <!-- Withdrawal (S-11): interrupting message boundary event on Beoordelen. On RegistratieIngetrokken
+         the task is cancelled and the process ends as "ingetrokken". -->
+    <boundaryEvent id="Ingetrokken" attachedToRef="Beoordelen" cancelActivity="true">
+      <messageEventDefinition messageRef="Message_Ingetrokken"/>
+    </boundaryEvent>
+
+    <sequenceFlow id="flow4" sourceRef="Ingetrokken" targetRef="endIngetrokken"/>
+
+    <endEvent id="endIngetrokken" name="Registratie ingetrokken"/>
   </process>
 
   <bpmndi:BPMNDiagram id="diagram">
@@ -57,6 +73,16 @@
         <omgdi:waypoint x="510" y="115"/>
         <omgdi:waypoint x="580" y="115"/>
       </bpmndi:BPMNEdge>
+      <bpmndi:BPMNShape id="s_ingetrokken" bpmnElement="Ingetrokken">
+        <omgdc:Bounds x="435" y="135" width="30" height="30"/>
+      </bpmndi:BPMNShape>
+      <bpmndi:BPMNShape id="s_endIngetrokken" bpmnElement="endIngetrokken">
+        <omgdc:Bounds x="435" y="220" width="30" height="30"/>
+      </bpmndi:BPMNShape>
+      <bpmndi:BPMNEdge id="e_flow4" bpmnElement="flow4">
+        <omgdi:waypoint x="450" y="165"/>
+        <omgdi:waypoint x="450" y="220"/>
+      </bpmndi:BPMNEdge>
     </bpmndi:BPMNPlane>
   </bpmndi:BPMNDiagram>
 </definitions>