feat(#13): S-12c-1 — behandel BFF auth + werkbak (ADR-0013) (#85)

## What & why

First half of **S-12c** (behandel-portal backend), per **ADR-0013** (decisions recorded in #84):

- **BFF multi-realm auth.** A second JWT bearer scheme (`medewerker`) alongside the default `digid` scheme. On validation it lifts Keycloak's `realm_access.roles` onto the principal, and a `behandelaar` policy (medewerker scheme + `behandelaar` role) gates `/behandel/*`. Self-service keeps the digid scheme.
- **Werkbak = Flowable tasks.** The domain `Werkbak` query reads the open `Beoordelen` tasks (§8.2, S-12b's `IUserTaskClient`) and enriches each with its aggregate's bsn + status; `GET /behandel/werkbak` (domain) is proxied by the BFF `GET /behandel/werkbak` behind the behandelaar policy. The read projection stays the anonymous openbaar model (no premature `IN_BEHANDELING`/personal-data plumbing — deferred in ADR-0008).

Behavior: `/behandel/werkbak` is **401** without a token, **403** for a medewerker lacking the role, **200 + werkbak** for a behandelaar.

**S-12c-2** (next): `POST /behandel/registrations/{id}/decide` → domain decision + complete the Flowable task.

## Definition of Done

- [x] Linked issue: #13 (umbrella, `refs`); closes the adr-proposal #84
- [x] Tests first; red → green per layer
- [x] Unit + acceptance green (`make unit`): domain 78, bff 23, acceptance 9 (+ acl/event-subscriber unaffected)
- [x] api-client `test` green; openapi.json regenerated (drift guard passes)
- [x] Mutation ≥ break(90): **domain 100%, bff 100%**
- [x] ADR-0013 added; `Keycloak__MedewerkerAuthority` wired into compose
- [ ] CI green (pending)

Part of #13. closes #84

Reviewed-on: #85

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;
+    }
+}