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>

infra/flowable/docker-compose.yml

@@ -0,0 +1,65 @@
+# Flowable (S-03): the flowable-rest engine on Postgres.
+# The registratie.bpmn model is deployed via the REST API on startup by flowable-init.
+#
+#   docker compose -f infra/flowable/docker-compose.yml up -d
+#   # REST API (basic auth) under http://localhost:8090/flowable-rest/service/
+#
+# Host port 8090 (8000/8001/8080/8180 are taken by OpenZaak/NRC/BFF/Keycloak).
+services:
+  flowable-db:
+    image: docker.io/library/postgres:16
+    environment:
+      POSTGRES_USER: flowable
+      POSTGRES_PASSWORD: flowable
+      POSTGRES_DB: flowable
+    volumes:
+      - flowable-db:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U flowable -d flowable"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+    networks: [cg]
+
+  flowable-rest:
+    image: docker.io/flowable/flowable-rest:latest
+    environment:
+      SPRING_DATASOURCE_DRIVER-CLASS-NAME: org.postgresql.Driver
+      SPRING_DATASOURCE_URL: jdbc:postgresql://flowable-db:5432/flowable
+      SPRING_DATASOURCE_USERNAME: flowable
+      SPRING_DATASOURCE_PASSWORD: flowable
+    ports:
+      - "8090:8080"
+    depends_on:
+      flowable-db:
+        condition: service_healthy
+    networks: [cg]
+
+  # Deploys workflows/registratie.bpmn via the REST API once flowable-rest is up.
+  # Idempotent: skips if a deployment named "registratie" already exists.
+  flowable-init:
+    image: docker.io/curlimages/curl:latest
+    restart: "no"
+    volumes:
+      - ../../workflows/registratie.bpmn:/work/registratie.bpmn:ro,z
+    command:
+      - sh
+      - -c
+      - |
+        base=http://flowable-rest:8080/flowable-rest/service/repository/deployments
+        until curl -sf -u rest-admin:test "$$base" >/dev/null 2>&1; do echo "waiting for flowable-rest..."; sleep 3; done
+        if curl -s -u rest-admin:test "$$base?name=registratie" | grep -q '"name":"registratie"'; then
+          echo "registratie already deployed; skip"
+        else
+          curl -sf -u rest-admin:test -F 'file=@/work/registratie.bpmn;filename=registratie.bpmn' "$$base" >/dev/null && echo "deployed registratie"
+        fi
+    depends_on:
+      flowable-rest:
+        condition: service_started
+    networks: [cg]
+
+volumes:
+  flowable-db:
+
+networks:
+  cg:

infra/flowable/verify.py

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+"""Smoke-check Flowable: the registratie process is deployed, and starting an
+instance parks it on the OpenZaakAanmaken external task. Stdlib only.
+"""
+import base64, json, sys, time, urllib.error, urllib.request
+
+BASE = "http://localhost:8090/flowable-rest/service"
+AUTH = "Basic " + base64.b64encode(b"rest-admin:test").decode()
+
+
+def call(method, path, payload=None):
+    data = json.dumps(payload).encode() if payload is not None else None
+    req = urllib.request.Request(BASE + path, data=data, method=method, headers={
+        "Authorization": AUTH, "Content-Type": "application/json", "Accept": "application/json"})
+    with urllib.request.urlopen(req, timeout=30) as r:
+        return r.status, json.loads(r.read() or "null")
+
+
+def main():
+    # 1. process definition deployed? (wait for the async init-container deploy)
+    defs = {"total": 0}
+    for _ in range(40):
+        _, defs = call("GET", "/repository/process-definitions?key=registratie")
+        if defs["total"] >= 1:
+            break
+        time.sleep(3)
+    assert defs["total"] >= 1, "registratie process definition not deployed"
+    print(f"process definition 'registratie' deployed (total={defs['total']})")
+
+    # 2. start an instance
+    st, pi = call("POST", "/runtime/process-instances", {"processDefinitionKey": "registratie"})
+    assert st == 201, f"start failed: {st} {pi}"
+    pid = pi["id"]
+    assert pi.get("ended") is False, "instance ended immediately — external task not reached"
+    print(f"started instance {pid} (ended={pi.get('ended')})")
+
+    # 3. waiting on the external task?
+    _, ex = call("GET", f"/runtime/executions?processInstanceId={pid}")
+    activities = [e.get("activityId") for e in ex["data"]]
+    assert "OpenZaakAanmaken" in activities, f"not waiting at OpenZaakAanmaken: {activities}"
+    print(f"instance is waiting at the external task: {activities}")
+
+    # 4. cleanup
+    try:
+        call("DELETE", f"/runtime/process-instances/{pid}")
+        print("cleaned up instance")
+    except urllib.error.HTTPError:
+        pass
+
+    print("flowable smoke OK")
+
+
+if __name__ == "__main__":
+    main()