feat(workflow): beoordeling escalation to teamlead after 14 days (S-14, closes #15) (#99)

## What & why

S-14: a beoordeling a behandelaar does not pick up within **14 days** escalates to the **teamlead**.

A non-interrupting `P14D` boundary timer on the `Beoordelen` user task fires an external-worker task
(`BeoordelingEscaleren`); the domain's escalation worker reassigns the still-open task's candidate group
from `behandelaar` to `teamlead`. The task keeps its identity — only who may claim it changes. The
escalation-via-external-worker decision is recorded in **ADR-0015** (proposal #98); it upholds §8.2
(the Workflow Client stays the only code that talks to Flowable) and keeps Flowable a stock image.

Closes #15

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the implementation.
- [x] Implementation makes the test pass; refactor commit if structure improved.
- [x] Conventional Commits referencing the issue (`refs #NN`).
- [x] CI green — all Gitea Actions jobs.
- [x] `docker compose up` from a fresh clone reaches green health checks within 3 minutes (no new services; escalation is additive to the domain worker).
- [x] Docs updated (ADR-0015, demo note).
- [x] ADR added (`docs/architecture/adr-0015-beoordeling-escalation.md`).
- [x] Demo note in `docs/demo-script.md`.

## How it was built (TDD)

- **Workflow Client** (`IBeoordelingEscalatieClient`): acquire `BeoordelingEscaleren` jobs → find the open `Beoordelen` task in the instance → add `teamlead`/remove `behandelaar` candidate group → complete the job. Red → green.
- **Escalation drain loop** (`BeoordelingEscalatieProcessor`) + hosted `BeoordelingEscalatiePump`, mirroring the OpenZaak worker. Red → green.
- **BPMN**: non-interrupting `P14D` boundary timer on `Beoordelen` → external task → escalation end.
- **Both branches** (escalate after timeout; no-op when completed in time) covered by the `Een beoordeling escaleren` acceptance scenarios + Workflow Client unit tests.
- **Live integration**: `verify-domain` fires the timer early via Flowable's management API and asserts the reassignment to teamlead.

## Notes for reviewers

- Interface segregation: escalation is on `IBeoordelingEscalatieClient`, separate from the OpenZaak worker's `IExternalWorkerClient`.
- Reassignment is two REST hops (add teamlead, remove behandelaar); idempotent on redelivery — see ADR-0015 consequences.
- Local checks green: domain unit tests (104), acceptance (13), `dotnet format --verify-no-changes`, Release build (0 errors), **domain mutation 96.69%** (break 90). The `run-domain-check.sh` escalation path is CI-verified on verify-stack (local full-stack run is constrained here).
- `BeoordelingEscalatiePump` excluded from mutation, mirroring the existing `OpenZaakJobPump` exclusion.

Reviewed-on: #99

services/domain/Big.Infrastructure/IExternalWorkerClient.cs

@@ -16,3 +16,23 @@ public interface IExternalWorkerClient
     /// <summary>Complete an acquired job, passing the opened zaak URL back into the process.</summary>
     Task CompleteOpenZaakJobAsync(string jobId, Uri zaakUrl, CancellationToken ct = default);
 }
+
+/// <summary>
+/// The escalation side of the Workflow Client (S-14): the <c>BeoordelingEscaleren</c> external-worker
+/// jobs parked by the 14-day boundary timer on <c>Beoordelen</c>, and the reassignment they drive.
+/// Kept separate from <see cref="IExternalWorkerClient"/> (interface segregation) so the OpenZaak
+/// worker never sees escalation. Implemented by <see cref="FlowableWorkflowClient"/> — the only code
+/// that talks to Flowable (§8.2, ADR-0015).
+/// </summary>
+public interface IBeoordelingEscalatieClient
+{
+    /// <summary>Acquire and lock up to <paramref name="maxJobs"/> <c>BeoordelingEscaleren</c> jobs.</summary>
+    Task<IReadOnlyList<EscalatieJob>> AcquireBeoordelingEscalatieJobsAsync(int maxJobs, CancellationToken ct = default);
+
+    /// <summary>Reassign the still-open <c>Beoordelen</c> task in the given process instance from the
+    /// behandelaar group to teamlead. Best-effort no-op if the task is no longer open.</summary>
+    Task ReassignBeoordelingToTeamleadAsync(string processInstanceId, CancellationToken ct = default);
+
+    /// <summary>Complete an acquired escalation job so its token reaches the escalation end event.</summary>
+    Task CompleteBeoordelingEscalatieJobAsync(string jobId, CancellationToken ct = default);
+}