feat(domain): implement the Flowable Workflow Client and ACL client (refs #6)

FlowableWorkflowClient speaks flowable-rest's REST API (Basic auth): start a
registratie process with the registrationId variable, acquire OpenZaakAanmaken
external-worker jobs and parse their registrationId, complete a job with the
zaakUrl variable — the contract verified against a live engine. AclHttpClient
POSTs the bsn to the ACL and returns the zaak URL. InMemoryRegistrationStore is
a concurrent-dictionary upsert. OpenZaakJobProcessor drains parked jobs, opening
a zaak per job and completing it, leaving failures for redelivery; OpenZaakJobPump
is the hosted polling shell that drives it on an interval (ADR-0009).

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

services/domain/Big.Infrastructure/OpenZaakJobProcessor.cs

@@ -15,12 +15,25 @@ public sealed class OpenZaakJobProcessor(
     OpenZaakWorker worker,
     ILogger<OpenZaakJobProcessor> logger)
 {
-    /// <summary>Process up to <paramref name="maxJobs"/> jobs. Returns the number completed.</summary>
+    /// <summary>Acquire and process up to <paramref name="maxJobs"/> jobs. Returns the number acquired.</summary>
     public async Task<int> PumpOnceAsync(int maxJobs, CancellationToken ct = default)
     {
-        // STUB (red).
-        await Task.CompletedTask;
-        _ = logger;
-        return 0;
+        var jobs = await client.AcquireOpenZaakJobsAsync(maxJobs, ct);
+
+        foreach (var job in jobs)
+        {
+            try
+            {
+                var zaakUrl = await worker.HandleAsync(job, ct);
+                await client.CompleteOpenZaakJobAsync(job.JobId, zaakUrl, ct);
+            }
+            catch (Exception ex)
+            {
+                // Leave the job un-completed: its lock expires and Flowable redelivers it (§8.6).
+                logger.LogError(ex, "OpenZaakAanmaken job {JobId} failed; leaving it for redelivery.", job.JobId);
+            }
+        }
+
+        return jobs.Count;
     }
 }