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

docs/demo-script.md

@@ -5,6 +5,28 @@ copy-pasteable walkthrough against a local `make up` stack.
 
 ---
 
+## S-15b — Beheer-portal: default-fill configuration editor (#131, ADR-0026)
+
+**Outcome:** a beheerder edits the ACL's ZGW **default-fill** values (bronorganisatie,
+verantwoordelijke organisatie, vertrouwelijkheidaanduiding) from the beheer portal, and the next zaak
+is stamped with the new values — no restart. Path: portal → BFF `GET/PUT /beheer/default-fill`
+(beheerder role) → ACL `GET/PUT /default-fill` → a runtime-mutable in-memory store the ACL reads per
+zaak (ADR-0026). The S-27 catalog-resolution keys stay static config (editing them would desync the
+zaaktype cache). Store is in-memory: an edit reverts to the configured env on restart.
+
+```bash
+make up
+# 1. Log in as bram-beheerder / test123 → "Default-fill" tab → change a value → Opslaan.
+open http://localhost:8143/default-fill
+#
+# 2. Automated: the ACL uses the current default-fill per zaak (unit) and the endpoints are behind the
+#    beheerder role (BFF unit):
+#    Acl.Tests → AclServiceTests.Opening_a_zaak_reflects_a_default_fill_update
+#    Bff.Tests → BeheerDefaultFillEndpointTests
+```
+
+---
+
 ## S-15a — Beheer-portal: read-only catalogus viewer (#130, ADR-0025)
 
 **Outcome:** a new **beheer** portal (medewerker realm, like behandel) shows the ZTC catalogus —