feat(workflow): Flowable up with registratie.bpmn external task (closes #4)

Add infra/flowable/docker-compose.yml (flowable-rest on Postgres, host :8090)
and workflows/registratie.bpmn — a minimal "Registratie ontvangen" process:
start -> external-worker task OpenZaakAanmaken -> end. A flowable-init
container deploys the model via the REST API on boot (idempotent: skips if
already deployed).

Add `make flowable-up/flowable-smoke/flowable-down`; flowable-smoke runs
infra/flowable/verify.py, which starts an instance and asserts it parks on
the OpenZaakAanmaken external task, then cleans up. Runbook included.

Verified clean-slate: down --volumes -> `make flowable-smoke` deploys on
boot, starts an instance, and confirms it waits at OpenZaakAanmaken.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workflows/registratie.bpmn

@@ -0,0 +1,48 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"
+             xmlns:flowable="http://flowable.org/bpmn"
+             xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI"
+             xmlns:omgdc="http://www.omg.org/spec/DD/20100524/DC"
+             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. -->
+  <process id="registratie" name="Registratie ontvangen" isExecutable="true">
+
+    <startEvent id="start" name="Registratie ontvangen"/>
+
+    <sequenceFlow id="flow1" sourceRef="start" targetRef="OpenZaakAanmaken"/>
+
+    <serviceTask id="OpenZaakAanmaken" name="OpenZaak aanmaken"
+                 flowable:type="external-worker"
+                 flowable:topic="OpenZaakAanmaken"/>
+
+    <sequenceFlow id="flow2" sourceRef="OpenZaakAanmaken" targetRef="end"/>
+
+    <endEvent id="end" name="Zaak aangemaakt"/>
+  </process>
+
+  <bpmndi:BPMNDiagram id="diagram">
+    <bpmndi:BPMNPlane id="plane" bpmnElement="registratie">
+      <bpmndi:BPMNShape id="s_start" bpmnElement="start">
+        <omgdc:Bounds x="100" y="100" width="30" height="30"/>
+      </bpmndi:BPMNShape>
+      <bpmndi:BPMNShape id="s_task" bpmnElement="OpenZaakAanmaken">
+        <omgdc:Bounds x="200" 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"/>
+      </bpmndi:BPMNShape>
+      <bpmndi:BPMNEdge id="e_flow1" bpmnElement="flow1">
+        <omgdi:waypoint x="130" y="115"/>
+        <omgdi:waypoint x="200" y="115"/>
+      </bpmndi:BPMNEdge>
+      <bpmndi:BPMNEdge id="e_flow2" bpmnElement="flow2">
+        <omgdi:waypoint x="320" y="115"/>
+        <omgdi:waypoint x="400" y="115"/>
+      </bpmndi:BPMNEdge>
+    </bpmndi:BPMNPlane>
+  </bpmndi:BPMNDiagram>
+</definitions>