feat(workflow): user-task client for beoordeling — werkbak, claim, complete (refs #13)

Adds the IUserTaskClient port and implements it on the Workflow Client (the only code
that talks to Flowable, §8.2): query open Beoordelen tasks (werkbak) with their
registrationId, claim a task for a behandelaar, and complete it carrying the besluit
into the process. Registered in DI for later wiring (S-12c).

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

services/domain/Big.Application/Ports.cs

@@ -36,6 +36,30 @@ public interface IAclClient
     Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default);
 }
 
+/// <summary>
+/// The port to the behandelaar's user tasks in the workflow engine (S-12). Implemented by the
+/// Workflow Client — the only code that talks to Flowable (§8.2). The application lists the open
+/// <c>Beoordelen</c> tasks (the werkbak), claims one for a behandelaar, and completes it with the
+/// decision; it never names Flowable's REST shapes.
+/// </summary>
+public interface IUserTaskClient
+{
+    /// <summary>The werkbak: the <c>Beoordelen</c> tasks awaiting a behandelaar, each with the
+    /// registration it belongs to.</summary>
+    Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default);
+
+    /// <summary>Claim a beoordeling task for a behandelaar (assigns it to them).</summary>
+    Task ClaimAsync(string taskId, string behandelaar, CancellationToken ct = default);
+
+    /// <summary>Complete a beoordeling task, carrying the decision into the process as the
+    /// <c>besluit</c> variable so the workflow can continue on the chosen branch.</summary>
+    Task CompleteBeoordelingAsync(string taskId, BeoordelingsBesluit besluit, CancellationToken ct = default);
+}
+
+/// <summary>A <c>Beoordelen</c> user task in the werkbak: the Flowable task id (needed to claim and
+/// complete it) and the registration it carries as a process variable.</summary>
+public sealed record BeoordelingTask(string TaskId, RegistrationId RegistrationId);
+
 /// <summary>
 /// Persistence port for the <see cref="Registration"/> aggregate. In-memory for the minimal slice
 /// (ADR-0009); an EF-backed store is a documented follow-up, and this port keeps that change additive.