feat(workflow): document-wait task + 30-day timeout cancellation (S-10a, closes #102) (#105)

## What & why

S-10a, the **workflow/timeout spine** of the (split) document-upload slice: the registratie process
now parks at a **`WachtOpDocumenten`** user task with an **interrupting `P30D` boundary timer**. When
the documents arrive the task completes and the process continues into the diploma routing (S-13) →
Beoordelen; if the 30 days lapse, the timer cancels the wait, runs a `RegistratieVerlopen`
external-worker task, and the domain expires the aggregate to a new terminal status **`Verlopen`**.
Backend only — the real upload trigger (portal → BFF → ACL → Documenten API) is S-10b (#103).

Closes #102

Mechanism recorded in **ADR-0017**; opened as proposal #104. Mirrors the S-14 escalation
(boundary-timer + external-worker) and S-11 withdrawal (interrupting cancel) patterns.

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the implementation (red→green pairs per layer).
- [x] Implementation makes the test pass.
- [x] Conventional Commits referencing the issue (`refs #102`).
- [ ] CI green — all Gitea Actions jobs (pending on this PR).
- [x] `docker compose up` health unaffected (no new services; deploy path unchanged).
- [x] Docs updated (ADR-0017, demo-script, BACKLOG split).
- [x] ADR added (`docs/architecture/adr-0017-document-wait-timeout-cancellation.md`).
- [x] Demo note in `docs/demo-script.md`.

## Notes for reviewers

- **Domain** (`Registration.Expire()` + `Verlopen`), **application** (`ExpireRegistrationWorker`),
  **infra** (`RegistratieVerlopenProcessor`/`Pump`, `IRegistratieVerlopenClient`, Flowable
  acquire/complete + `CompleteDocumentWaitAsync`) — the timeout counterpart to the OpenZaak/escalation
  worker trios; idempotent per §8.6.
- **BPMN** verified live against a `flowable-rest` probe: complete `WachtOpDocumenten` → routes to
  Beoordelen; fire the P30D timer → `RegistratieVerlopen` job (carrying `registrationId`) + the wait
  task cancelled. `verify-domain` exercises both branches in-stack (completes the wait in every existing
  block; fires the timer and asserts `Verlopen` in a new block).
- **Scope boundary:** on expiry the aggregate goes `Verlopen` and the process ends, but the ZGW *zaak*
  is not yet set to a cancellation status — that needs a new ACL method + statustype seeding and is
  folded into S-10b (noted in ADR-0017).
- `CompleteDocumentWaitAsync` is built and HTTP-tested here but not yet called from a domain endpoint;
  S-10b wires the upload trigger to it.

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Reviewed-on: #105

services/domain/Big.Infrastructure/IExternalWorkerClient.cs

@@ -36,3 +36,19 @@ public interface IBeoordelingEscalatieClient
     /// <summary>Complete an acquired escalation job so its token reaches the escalation end event.</summary>
     Task CompleteBeoordelingEscalatieJobAsync(string jobId, CancellationToken ct = default);
 }
+
+/// <summary>
+/// The document-timeout side of the Workflow Client (S-10a): the <c>RegistratieVerlopen</c>
+/// external-worker jobs parked by the 30-day boundary timer on <c>WachtOpDocumenten</c>. Kept separate
+/// from the other worker ports (interface segregation) so neither the OpenZaak nor escalation worker
+/// sees expiry. Implemented by <see cref="FlowableWorkflowClient"/> — the only code that talks to
+/// Flowable (§8.2, ADR-0017).
+/// </summary>
+public interface IRegistratieVerlopenClient
+{
+    /// <summary>Acquire and lock up to <paramref name="maxJobs"/> <c>RegistratieVerlopen</c> jobs.</summary>
+    Task<IReadOnlyList<RegistratieVerlopenJob>> AcquireRegistratieVerlopenJobsAsync(int maxJobs, CancellationToken ct = default);
+
+    /// <summary>Complete an acquired expiry job so its token reaches the <c>endVerlopen</c> end event.</summary>
+    Task CompleteRegistratieVerlopenJobAsync(string jobId, CancellationToken ct = default);
+}