arch(domain): ADR-0009 external-task job-worker pattern (refs #6, #60)

The Domain Service drives the OpenZaakAanmaken external-worker task as a
hosted job worker (PRD §36): POST /registrations starts the registratie
process and returns; a polling worker acquires the job, opens a zaak via
the ACL (§8.1), attaches the zaak URL to the aggregate, and completes the
job. The Workflow Client is the only Flowable client (§8.2); the worker
logic is an Application service over ports. Registration state is in-memory
for the minimal slice (the read path is the projection, S-06).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mkdocs.yml

@@ -30,6 +30,7 @@ nav:
       - "ADR-0006: ACL integration test provisioning": architecture/adr-0006-integration-test-provisioning.md
       - "ADR-0007: OpenZaak → NRC notification wiring": architecture/adr-0007-notification-wiring.md
       - "ADR-0008: Read projection store": architecture/adr-0008-read-projection-store.md
+      - "ADR-0009: External-task job worker": architecture/adr-0009-external-task-job-worker.md
   - Working in Gitea: gitea-workflow.md
   - Demo script: demo-script.md
   - Runbooks: