feat(workflow): Workflow Client reassigns beoordeling to teamlead (S-14, refs #15)

Implement the BeoordelingEscaleren escalation capability on the Flowable
Workflow Client: acquire escalation jobs, find the still-open Beoordelen task in
the instance and swap its candidate group behandelaar -> teamlead via task
identity links, and complete the job. Segregated onto IBeoordelingEscalatieClient
so the OpenZaak worker is unaffected (ADR-0015).

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

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);
+}