feat(domain): RegistratieVerlopen worker + processor expire on document timeout (refs #102)

Adds RegistratieVerlopenJob, IRegistratieVerlopenClient, the ExpireRegistrationWorker
application handler, and the RegistratieVerlopenProcessor drain loop — the timeout
counterpart to the OpenZaak/escalation worker trios.

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

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);
+}