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/InMemoryRegistrationStore.cs

@@ -15,13 +15,11 @@ public sealed class InMemoryRegistrationStore : IRegistrationStore
 
     public Task SaveAsync(Registration registration, CancellationToken ct = default)
     {
-        // STUB (red).
+        ArgumentNullException.ThrowIfNull(registration);
+        _byId[registration.Id] = registration;
         return Task.CompletedTask;
     }
 
     public Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default)
-    {
-        // STUB (red).
-        return Task.FromResult<Registration?>(null);
-    }
+        => Task.FromResult(_byId.GetValueOrDefault(id));
 }