feat(#13): S-12c-2 — behandel decide → domain + complete workflow task (#86)

## What & why

Second half of **S-12c** (behandel-portal backend), completing the decision path per **ADR-0013**:

- **Domain:** `BeoordeelRegistratie` now, after applying the decision (aggregate + ACL for approval), **completes the open Flowable `Beoordelen` task** for that registration (found by registrationId) with the besluit, so the workflow advances. No open task → the decision still stands (completes nothing); idempotent.
- **BFF:** `POST /behandel/registrations/{id}/decide` behind the medewerker/`behandelaar` policy, forwarding `goedkeuren`/`afwijzen` to the domain. Validates the besluit vocabulary (400 on unknown) without troubling the domain.

Behavior: decide is **401** without a token, **403** without the role, **400** for an unknown besluit, **204** (forwarded) for a behandelaar.

This completes the behandel backend. **S-12d** (the Angular behandel-portal + Playwright e2e) closes umbrella #13 and retires the temporary `/approve`.

## Definition of Done

- [x] Linked issue: #13 (umbrella, `refs`)
- [x] Tests first; red → green per layer
- [x] Unit + acceptance green (`make unit`): domain 79, bff 27, acceptance 9 (acl/event-subscriber unaffected)
- [x] Beoordeling acceptance scenario asserts task completion (goedkeuren + afwijzen)
- [x] openapi.json + api-client regenerated (drift guard passes)
- [x] Mutation ≥ break(90): **domain 100%, bff 100%**
- [ ] CI green (pending)

Part of #13.

Reviewed-on: #86

services/bff/Bff.Api/DownstreamClients.cs

@@ -24,6 +24,9 @@ public interface IDomainClient
 
     /// <summary>The behandelaar's werkbak — registrations awaiting beoordeling.</summary>
     Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default);
+
+    /// <summary>Apply a behandelaar's decision (<c>goedkeuren</c>/<c>afwijzen</c>) to a registration.</summary>
+    Task DecideAsync(string registrationId, string besluit, CancellationToken ct = default);
 }
 
 /// <summary>Port to the read projection.</summary>
@@ -47,6 +50,13 @@ public sealed class DomainClient(HttpClient http) : IDomainClient
     public async Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => await http.GetFromJsonAsync<List<WerkbakItem>>("behandel/werkbak", ct) ?? [];
 
+    public async Task DecideAsync(string registrationId, string besluit, CancellationToken ct = default)
+    {
+        using var response = await http.PostAsJsonAsync(
+            $"registrations/{registrationId}/decide", new { besluit }, ct);
+        response.EnsureSuccessStatusCode();
+    }
+
     private sealed record DomainResponse(string RegistrationId, string Status, string? ZaakUrl);
 }