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/acl/Acl.Api/Program.cs

@@ -33,6 +33,15 @@ builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
 builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
     .GetSection("Acl:OpenZaak").Get<OpenZaakOptions>()
     ?? throw new InvalidOperationException("Missing configuration section 'Acl:OpenZaak'"));
+// The default-fill values are held in a runtime-mutable store (S-15b, ADR-0026), seeded from the
+// configured Acl:Defaults. The beheer portal edits it; the worker reads it per zaak. The S-27
+// resolution keys stay on AclDefaults (static) — see DefaultFillSettings.
+builder.Services.AddSingleton<IDefaultFillStore>(sp =>
+{
+    var d = sp.GetRequiredService<AclDefaults>();
+    return new InMemoryDefaultFillStore(
+        new DefaultFillSettings(d.Bronorganisatie, d.VerantwoordelijkeOrganisatie, d.Vertrouwelijkheidaanduiding));
+});
 builder.Services.AddHttpClient<IZaakGateway, OpenZaakGateway>();
 // Singleton so the resolved zaaktype/informatieobjecttype URLs are cached across requests (S-27).
 builder.Services.AddSingleton<IZaaktypeCatalog, CachedZaaktypeCatalog>();
@@ -91,6 +100,22 @@ app.MapPost("/documenten", async (StoreDocumentRequest body, AclService acl, Can
 app.MapGet("/catalogi/zaaktypen", async (AclService acl, CancellationToken ct) =>
     Results.Ok(await acl.ListZaaktypenAsync(ct)));
 
+// Read the current default-fill settings (beheer config viewer, S-15b).
+app.MapGet("/default-fill", (AclService acl) => Results.Ok(acl.GetDefaultFill()));
+
+// Update the default-fill settings from the beheer portal (S-15b). Behind beheerder authorization at
+// the BFF; the ACL validates the values are present (the three ZGW-mandatory fields).
+app.MapPut("/default-fill", (DefaultFillSettings body, AclService acl) =>
+{
+    if (string.IsNullOrWhiteSpace(body.Bronorganisatie) ||
+        string.IsNullOrWhiteSpace(body.VerantwoordelijkeOrganisatie) ||
+        string.IsNullOrWhiteSpace(body.Vertrouwelijkheidaanduiding))
+        return Results.BadRequest(new { error = "bronorganisatie, verantwoordelijkeOrganisatie and vertrouwelijkheidaanduiding are all required." });
+
+    acl.UpdateDefaultFill(body);
+    return Results.NoContent();
+});
+
 app.Run();
 
 public sealed record OpenZaakRequest(string Bsn, string Reference);