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/AclHttpClient.cs

@@ -1,3 +1,5 @@
+using System.Net.Http.Json;
+using System.Text.Json.Serialization;
 using Big.Application;
 
 namespace Big.Infrastructure;
@@ -9,9 +11,18 @@ namespace Big.Infrastructure;
 /// </summary>
 public sealed class AclHttpClient(HttpClient http, AclOptions options) : IAclClient
 {
-    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    public async Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
     {
-        // STUB (red).
-        return Task.FromResult(new Uri("about:blank"));
+        using var response = await http.PostAsJsonAsync(
+            new Uri(options.BaseUrl, "zaken"), new OpenZaakRequest(bsn), ct);
+        response.EnsureSuccessStatusCode();
+
+        var opened = await response.Content.ReadFromJsonAsync<OpenZaakResponse>(ct)
+            ?? throw new InvalidOperationException("The ACL returned an empty zaak response.");
+        return new Uri(opened.ZaakUrl);
     }
+
+    private sealed record OpenZaakRequest([property: JsonPropertyName("bsn")] string Bsn);
+
+    private sealed record OpenZaakResponse([property: JsonPropertyName("zaakUrl")] string ZaakUrl);
 }