feat(portal-beheer): ACL default-fill configuration editor (closes #131) (#138)

## What & why

S-15b, second of the S-15 (#16) split, on top of S-15a (#133). A beheerder edits the ACL's ZGW **default-fill** values from the beheer portal, and the next zaak is stamped with the new values — no restart.

Closes #131

### The vertical

portal → BFF `GET/PUT /beheer/default-fill` (medewerker realm + `beheerder` role) → ACL `GET/PUT /default-fill` → a runtime-mutable in-memory store the ACL reads **per zaak**.

- **ACL**: `IDefaultFillStore` / `InMemoryDefaultFillStore` (thread-safe, seeded from `Acl:Defaults`); `AclService` reads `fill.Current` per zaak (not cached at construction); `GET`/`PUT /default-fill` with required-field validation.
- **BFF**: `IAclClient` gains `GetDefaultFillAsync`/`UpdateDefaultFillAsync`; `GET`/`PUT /beheer/default-fill` behind the `beheerder` policy. OpenAPI + generated client regenerated.
- **Frontend**: a *Default-fill* editor page in the beheer app (load → edit → save, with saved/failure states) + nav between Catalogus and Default-fill.

### Scope decision → ADR-0026

Only the **three ZGW fill fields** (bronorganisatie, verantwoordelijke organisatie, vertrouwelijkheidaanduiding) are editable. The S-27 catalog-resolution keys stay **static config** — editing them would desync the zaaktype-URL cache (ADR-0021), and they're catalogus wiring, not "default fill". The store is **in-memory** (seeded from config): an edit reverts on restart. That's the reference-app-appropriate ceiling (no DB added to the stateless ACL); upgrade path documented. Recorded in **ADR-0026**.

## Verified locally

lint (`dotnet format`) ✓ · .NET unit — acl 60 / bff 45 / domain 152 / event-subscriber 19 / acceptance 17 ✓ · frontend lint+test (8 projects) ✓ · beheer build ✓. Clean full-solution build (caught + fixed the acceptance `AclService` ctor drift). TDD red→green per layer (ACL store, ACL endpoints, BFF, frontend).

## Definition of Done

- [x] Failing test committed before each implementation (red→green per layer).
- [x] Conventional Commits referencing #131.
- [ ] CI green — see note below.
- [x] Docs: ADR-0026 + S-15b demo note.
- [x] Demo note in `docs/demo-script.md`.

## Note on CI

The bulk validates in the fast jobs (lint/build/unit/frontend/mutation). The **verify-stack e2e** (incl. the new `default-fill.spec.ts`) can't go green until the pre-existing **verify-stack bring-up failure on the 1.27/2.0.0 runner** is resolved (that fails on plain `main` too — unrelated to this PR). Additive change; no existing e2e touched.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #138

services/bff/Bff.Api/DownstreamClients.cs

@@ -59,12 +59,26 @@ public interface IProjectionClient
 /// internal reference, not shown in the portal.</summary>
 public sealed record BeheerZaaktype(string Identificatie, string Omschrijving);
 
-/// <summary>Port to the ACL for read-only catalogus queries (beheer portal, S-15a). The BFF reaches the
-/// ACL directly for this read: the catalogus isn't a domain concern, and the ACL is the only code
-/// allowed to read the ZGW Catalogi API (§8.1, ADR-0025).</summary>
+/// <summary>The ACL default-fill settings the beheer portal reads + edits (S-15b): the three ZGW-mandatory
+/// fields the ACL stamps on every zaak (ADR-0003).</summary>
+public sealed record BeheerDefaultFill(
+    string Bronorganisatie,
+    string VerantwoordelijkeOrganisatie,
+    string Vertrouwelijkheidaanduiding);
+
+/// <summary>Port to the ACL for beheer queries (beheer portal). The BFF reaches the ACL directly: these
+/// aren't a domain concern, and the ACL is the only code allowed to read/own the ZGW-facing config
+/// (§8.1, ADR-0025).</summary>
 public interface IAclClient
 {
+    /// <summary>The published catalogus zaaktypen, read-only (S-15a).</summary>
     Task<IReadOnlyList<BeheerZaaktype>> GetZaaktypenAsync(CancellationToken ct = default);
+
+    /// <summary>The current default-fill settings (S-15b).</summary>
+    Task<BeheerDefaultFill> GetDefaultFillAsync(CancellationToken ct = default);
+
+    /// <summary>Replace the default-fill settings (S-15b).</summary>
+    Task UpdateDefaultFillAsync(BeheerDefaultFill settings, CancellationToken ct = default);
 }
 
 /// <summary>Calls the Domain Service's <c>POST /registrations</c>.</summary>
@@ -141,4 +155,14 @@ public sealed class AclClient(HttpClient http) : IAclClient
 {
     public async Task<IReadOnlyList<BeheerZaaktype>> GetZaaktypenAsync(CancellationToken ct = default)
         => await http.GetFromJsonAsync<List<BeheerZaaktype>>("catalogi/zaaktypen", ct) ?? [];
+
+    public async Task<BeheerDefaultFill> GetDefaultFillAsync(CancellationToken ct = default)
+        => await http.GetFromJsonAsync<BeheerDefaultFill>("default-fill", ct)
+           ?? throw new InvalidOperationException("The ACL returned an empty default-fill response.");
+
+    public async Task UpdateDefaultFillAsync(BeheerDefaultFill settings, CancellationToken ct = default)
+    {
+        using var response = await http.PutAsJsonAsync("default-fill", settings, ct);
+        response.EnsureSuccessStatusCode();
+    }
 }

services/bff/Bff.Api/Program.cs

@@ -228,6 +228,25 @@ app.MapGet("/beheer/catalogi/zaaktypen", async (IAclClient acl, CancellationToke
     .Produces(StatusCodes.Status401Unauthorized)
     .Produces(StatusCodes.Status403Forbidden);
 
+// Beheer default-fill config (S-15b): read + edit the ACL's default-fill values. Behind medewerker-
+// realm + beheerder authorization; the BFF proxies the ACL (ADR-0025). The ACL validates the values.
+app.MapGet("/beheer/default-fill", async (IAclClient acl, CancellationToken ct) =>
+    Results.Ok(await acl.GetDefaultFillAsync(ct)))
+    .RequireAuthorization(BeheerAuth.Policy)
+    .Produces<BeheerDefaultFill>(StatusCodes.Status200OK)
+    .Produces(StatusCodes.Status401Unauthorized)
+    .Produces(StatusCodes.Status403Forbidden);
+
+app.MapPut("/beheer/default-fill", async (BeheerDefaultFill body, IAclClient acl, CancellationToken ct) =>
+{
+    await acl.UpdateDefaultFillAsync(body, ct);
+    return Results.NoContent();
+})
+    .RequireAuthorization(BeheerAuth.Policy)
+    .Produces(StatusCodes.Status204NoContent)
+    .Produces(StatusCodes.Status401Unauthorized)
+    .Produces(StatusCodes.Status403Forbidden);
+
 app.Run();
 
 /// <summary>The behandelaar's decision on a registration.</summary>