test(domain): Workflow Client, ACL client, store and job processor (refs #6)

Failing infrastructure unit tests (stub HttpMessageHandler, fakes):
- FlowableWorkflowClient starts a process with the registrationId variable and
  returns the instance id; acquires OpenZaakAanmaken jobs (topic/workerId/lock)
  and parses their registrationId; completes a job with the zaakUrl variable —
  request URIs match flowable-rest's service/ and external-job-api/ paths.
- AclHttpClient POSTs the bsn to the ACL and returns the zaak URL.
- InMemoryRegistrationStore saves/reads/upserts by id.
- OpenZaakJobProcessor acquires, opens a zaak, completes the job; leaves a failing
  job uncompleted for redelivery; polls harmlessly when idle.

Adapters are stubs so the tests compile and fail on their assertions; the green
commit implements them against the REST contract verified on a live Flowable.

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

services/domain/Big.Infrastructure/Options.cs

@@ -0,0 +1,29 @@
+namespace Big.Infrastructure;
+
+/// <summary>Configuration for the Flowable Workflow Client. <see cref="BaseUrl"/> is the flowable-rest
+/// root and must end with a slash (e.g. <c>http://flowable-rest:8080/flowable-rest/</c>) so the
+/// <c>service/…</c> and <c>external-job-api/…</c> sub-paths resolve correctly.</summary>
+public sealed class FlowableOptions
+{
+    public Uri BaseUrl { get; set; } = null!;
+    public string Username { get; set; } = "";
+    public string Password { get; set; } = "";
+
+    /// <summary>The id this worker locks jobs under, so two workers don't process the same job.</summary>
+    public string WorkerId { get; set; } = "big-domain-worker";
+
+    /// <summary>How long an acquired job stays locked to this worker (ISO-8601 duration).</summary>
+    public string LockDuration { get; set; } = "PT5M";
+
+    /// <summary>How many OpenZaakAanmaken jobs to acquire per poll.</summary>
+    public int MaxJobsPerPoll { get; set; } = 5;
+
+    /// <summary>How often the worker polls Flowable for new jobs.</summary>
+    public TimeSpan PollInterval { get; set; } = TimeSpan.FromSeconds(2);
+}
+
+/// <summary>Configuration for the ACL HTTP client. <see cref="BaseUrl"/> is the ACL service root.</summary>
+public sealed class AclOptions
+{
+    public Uri BaseUrl { get; set; } = null!;
+}