feat(domain): werkbak query + GET /behandel/werkbak (refs #13)

The Werkbak use-case reads the open Beoordelen tasks (§8.2) and enriches each with
its aggregate's bsn + status; exposed as GET /behandel/werkbak for the BFF to proxy.

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

services/domain/Big.Application/Werkbak.cs

@@ -0,0 +1,32 @@
+namespace Big.Application;
+
+/// <summary>One row of the behandelaar's werkbak: a registration awaiting beoordeling, with the
+/// public-facing reference (its id) plus the bsn and status a behandelaar needs to triage it.</summary>
+public sealed record WerkbakItem(string RegistrationId, string Bsn, string Status);
+
+/// <summary>
+/// The werkbak query (S-12c): the registrations awaiting a behandelaar's beoordeling. It reads the
+/// open <c>Beoordelen</c> tasks from the workflow engine (§8.2, via <see cref="IUserTaskClient"/>) —
+/// the authoritative set of work items — and enriches each with its aggregate (bsn + status). A task
+/// whose registration the domain doesn't know is skipped rather than invented.
+/// </summary>
+public sealed class Werkbak(IUserTaskClient tasks, IRegistrationStore store)
+{
+    public async Task<IReadOnlyList<WerkbakItem>> GetAsync(CancellationToken ct = default)
+    {
+        var open = await tasks.GetOpenBeoordelingenAsync(ct);
+
+        var items = new List<WerkbakItem>(open.Count);
+        foreach (var task in open)
+        {
+            var registration = await store.GetAsync(task.RegistrationId, ct);
+            if (registration is null)
+                continue;
+
+            items.Add(new WerkbakItem(
+                registration.Id.ToString(), registration.Bsn, registration.Status.ToString()));
+        }
+
+        return items;
+    }
+}