feat(#13): S-12b — Workflow Client user-tasks + Beoordelen userTask (#83)

## What & why

Second sub-slice of **S-12 (#13)** — the **Workflow Client gains behandelaar user-task operations**, and the process model gains the beoordeling step.

- **BPMN:** `registratie.bpmn` now parks at a `Beoordelen` **userTask** (candidate group `behandelaar`) after `OpenZaakAanmaken`; `registrationId` rides along as a process variable so the werkbak can correlate each task to its aggregate.
- **Workflow Client** (`IUserTaskClient`, the only code that talks to Flowable §8.2):
  - `GetOpenBeoordelingenAsync()` — the werkbak (open `Beoordelen` tasks + their `registrationId`)
  - `ClaimAsync(taskId, behandelaar)`
  - `CompleteBeoordelingAsync(taskId, besluit)` — carries the decision into the process as the `besluit` variable
- **Live integration:** `verify-domain` now drives the full user-task lifecycle against a real Flowable — after the worker opens the zaak, it polls for the task, claims it as `merel-behandelaar`, completes it (`goedkeuren`), and asserts the process finishes. This proves the exact REST contract (`service/runtime/tasks/query` + `…/{id}` claim/complete) the client depends on.

The walking skeleton is unaffected: the temporary `/approve` path still sets the zaak status directly; wiring the domain decision to *complete this task* (and driving the werkbak from the BFF) lands in **S-12c**.

## Definition of Done

- [x] Linked issue: #13 (umbrella; `refs`, does not close)
- [x] Tests first; red → green
- [x] Unit + acceptance green (`make unit`): domain 76, acceptance 9 (acl/event-subscriber/bff unaffected)
- [x] Mutation ≥ break(90): **domain 100%** (killed the new survivors *and* the pre-existing `FlowableWorkflowClient` baseline)
- [x] Live Flowable user-task lifecycle asserted in `verify-domain`
- [ ] CI green (pending)

Part of #13.

Reviewed-on: #83

workflows/registratie.bpmn

@@ -6,9 +6,12 @@
              xmlns:omgdi="http://www.omg.org/spec/DD/20100524/DI"
              targetNamespace="http://respellion.nl/big">
 
-  <!-- S-03: minimal "Registratie ontvangen" flow.
-       start -> external-worker task (OpenZaakAanmaken) -> end.
-       The external task is handled by the Workflow Client / ACL in later slices. -->
+  <!-- Registratie ontvangen flow.
+       start -> external-worker task (OpenZaakAanmaken) -> Beoordelen (behandelaar user task) -> end.
+       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. -->
   <process id="registratie" name="Registratie ontvangen" isExecutable="true">
 
     <startEvent id="start" name="Registratie ontvangen"/>
@@ -19,9 +22,13 @@
                  flowable:type="external-worker"
                  flowable:topic="OpenZaakAanmaken"/>
 
-    <sequenceFlow id="flow2" sourceRef="OpenZaakAanmaken" targetRef="end"/>
+    <sequenceFlow id="flow2" sourceRef="OpenZaakAanmaken" targetRef="Beoordelen"/>
 
-    <endEvent id="end" name="Zaak aangemaakt"/>
+    <userTask id="Beoordelen" name="Beoordelen" flowable:candidateGroups="behandelaar"/>
+
+    <sequenceFlow id="flow3" sourceRef="Beoordelen" targetRef="end"/>
+
+    <endEvent id="end" name="Registratie beoordeeld"/>
   </process>
 
   <bpmndi:BPMNDiagram id="diagram">
@@ -32,8 +39,11 @@
       <bpmndi:BPMNShape id="s_task" bpmnElement="OpenZaakAanmaken">
         <omgdc:Bounds x="200" y="85" width="120" height="60"/>
       </bpmndi:BPMNShape>
+      <bpmndi:BPMNShape id="s_beoordelen" bpmnElement="Beoordelen">
+        <omgdc:Bounds x="390" y="85" width="120" height="60"/>
+      </bpmndi:BPMNShape>
       <bpmndi:BPMNShape id="s_end" bpmnElement="end">
-        <omgdc:Bounds x="400" y="100" width="30" height="30"/>
+        <omgdc:Bounds x="580" y="100" width="30" height="30"/>
       </bpmndi:BPMNShape>
       <bpmndi:BPMNEdge id="e_flow1" bpmnElement="flow1">
         <omgdi:waypoint x="130" y="115"/>
@@ -41,7 +51,11 @@
       </bpmndi:BPMNEdge>
       <bpmndi:BPMNEdge id="e_flow2" bpmnElement="flow2">
         <omgdi:waypoint x="320" y="115"/>
-        <omgdi:waypoint x="400" y="115"/>
+        <omgdi:waypoint x="390" y="115"/>
+      </bpmndi:BPMNEdge>
+      <bpmndi:BPMNEdge id="e_flow3" bpmnElement="flow3">
+        <omgdi:waypoint x="510" y="115"/>
+        <omgdi:waypoint x="580" y="115"/>
       </bpmndi:BPMNEdge>
     </bpmndi:BPMNPlane>
   </bpmndi:BPMNDiagram>