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/OpenZaakJobPump.cs

@@ -0,0 +1,48 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+
+namespace Big.Infrastructure;
+
+/// <summary>
+/// The hosted polling loop of the external-task job worker (ADR-0009): on an interval it resolves a
+/// scoped <see cref="OpenZaakJobProcessor"/> and asks it to drain the parked <c>OpenZaakAanmaken</c>
+/// jobs. A deliberately thin shell — all acquire/process/complete logic lives in the processor, which
+/// is unit-tested; this class only owns the timer, the per-tick scope, and loop resilience.
+/// </summary>
+public sealed class OpenZaakJobPump(
+    IServiceScopeFactory scopeFactory,
+    FlowableOptions options,
+    ILogger<OpenZaakJobPump> logger) : BackgroundService
+{
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                using var scope = scopeFactory.CreateScope();
+                var processor = scope.ServiceProvider.GetRequiredService<OpenZaakJobProcessor>();
+                await processor.PumpOnceAsync(options.MaxJobsPerPoll, stoppingToken);
+            }
+            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
+            {
+                break;
+            }
+            catch (Exception ex)
+            {
+                // A transient fault (e.g. Flowable briefly unreachable) must not kill the loop.
+                logger.LogError(ex, "OpenZaakAanmaken job poll failed; retrying after the poll interval.");
+            }
+
+            try
+            {
+                await Task.Delay(options.PollInterval, stoppingToken);
+            }
+            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
+            {
+                break;
+            }
+        }
+    }
+}