feat(backend): expand stamdata + OpenZaak-ready cases seam (WP-49)

Stamdata: add beroepen, opleidingen (temporal), and specialismen tables to the
schema-driven catalog (zero UI code). opleidingen.beroep and specialismen.beroep
both reference beroepen.code — the first stamdata->stamdata references, enforced by
two new StamdataRef entries in the CI gate.

OpenZaak/ZGW (WP-49, slice 1 — read-only zaken): introduce IZaakSource as the cases
read seam. Default LocalZaakSource reads the local SQLite store (offline); an
OpenZaakZaakSource (Zgw/ client: HS256 per-call JWT, ZGW->existing-DTO mapper,
paginating HTTP source) is selected behind Zgw:Enabled (default false). The FE never
changes — same ApplicationSummaryDto, no api-client drift. Unit-tested with fixtures
+ a stub HttpMessageHandler; no live OpenZaak needed.

Docs: ADR-0005, reference/openzaak-integration.md, WP-49..52 roadmap, stamdata.md
update, README index rows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/reference/architecture/0005-openzaak-behind-bff.md

@@ -0,0 +1,69 @@
+# ADR-0005 — OpenZaak (ZGW APIs) behind the BFF
+
+Status: Accepted · Date: 2026-07-24
+
+## Context
+
+The POC serves cases (aanvragen) from a local SQLite store. To grow toward production it must
+be able to source cases from a real Dutch **Zaakgericht Werken (ZGW)** backend — **OpenZaak**,
+the VNG reference implementation. ZGW is not one API but five separate services (Zaken/ZRC,
+Documenten/DRC, Catalogi/ZTC, Besluiten/BRC, Notificaties/NRC), each on its own base URL, with
+traits that make raw responses unfit to hand to a browser:
+
+- resources are identified by **full URLs**, not bare ids;
+- references between resources are **URLs into other services** (a zaak's `zaaktype` lives in
+  Catalogi), so a single screen means joining across services;
+- lists use a uniform `{count,next,previous,results}` pagination envelope;
+- auth is a short-lived **HS256 JWT** signed with a client secret (no OAuth refresh), which
+  OpenZaak rejects an hour past `iat`.
+
+Two constraints shaped the decision: the **frontend must not change** (BFF-lite, ADR-0001 —
+the FE renders decision DTOs and never recomputes rules), and the POC must **still run fully
+offline** (no OpenZaak needed for local dev/CI).
+
+The backend, however, had **no data-access abstraction** — endpoints called concrete static
+stores directly — and no outbound HTTP or JWT machinery. So there was no injection point to
+swap a data source behind.
+
+## Options
+
+1. **FE talks to OpenZaak directly.** Rejected: leaks ZGW shapes + the client secret to the
+   browser, and contradicts BFF-lite.
+2. **Rewrite the static stores in place to call OpenZaak.** Rejected: no seam, no offline mode,
+   all-or-nothing, untestable without a live server.
+3. **Introduce a data-source interface behind the existing DTO contract, select the
+   implementation by config.** Chosen.
+
+## Decision
+
+Put the OpenZaak anti-corruption layer **in the .NET BFF**, never in the browser. Introduce a
+per-domain source interface (starting with `IZaakSource` for the cases read path) whose default
+implementation reads the local SQLite store and whose alternate implementation calls OpenZaak —
+selected by a config flag (`Zgw:Enabled`, default false). Each implementation maps into the
+**existing** wire DTO (`ApplicationSummaryDto`), so the `/api/v1` contract and the FE are
+untouched. The BFF holds the client secret and **mints a fresh JWT per outbound call**.
+
+This is deliberately a **thin vertical slice** (read-only zaken, WP-49); create/documents/
+notifications follow the same seam in later slices (WP-50/51/52) rather than being scaffolded
+up front — the migration stance ADR-0001 already prescribes.
+
+## Consequences
+
+- **+** The FE is production-ready as-is: swapping to OpenZaak is backend-only, behind one
+  config flag, with zero DTO/api-client drift. The POC still runs offline (default = local).
+- **+** The seam is unit-testable without a live server: the JWT minter, the ZGW→DTO mapper,
+  and the paginating source are all covered with fixtures + a stub `HttpMessageHandler`.
+- **+** URL-as-identity and cross-service joins are contained in one mapper; nothing downstream
+  sees a ZGW shape.
+- **−** Only the cases **read** path has a source interface today; other endpoints still call
+  static stores directly. Each future slice introduces its own seam as needed (not a big-bang
+  repository refactor).
+- **−** `IZaakSource` is synchronous (matching the existing sync endpoint + local store), so
+  `OpenZaakZaakSource` does sync-over-async; fine under ASP.NET Core (no sync-context), to be
+  made async if OpenZaak becomes the default. Marked with a `ponytail:` note at the call site.
+- **Shipped with this ADR (WP-49):** `IZaakSource` + `LocalZaakSource` (default) +
+  `OpenZaakZaakSource` (config-gated), the `Zgw/` client (`ZgwOptions`, `ZgwTokenProvider`,
+  `ZgwZaakMapper`), and the reference guide [openzaak-integration.md](../openzaak-integration.md).
+- **Deferred:** real inbound OIDC/JWT auth (still header-stubbed), create-zaak (WP-50),
+  Documenten/DRC upload + link (WP-51), Notificaties/NRC webhooks (WP-52), adding OpenZaak to
+  docker-compose.

docs/reference/openzaak-integration.md

@@ -0,0 +1,88 @@
+# OpenZaak / ZGW integration — how the BFF connects (& how to extend)
+
+How the BFF sources cases from a real **OpenZaak** (ZGW APIs) while the frontend stays
+unchanged. For the _why_, see [ADR-0005](architecture/0005-openzaak-behind-bff.md); this page
+is _how the seam is built and how to add the next slice_. Built in
+[WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (read-only zaken).
+
+## The one rule: OpenZaak sits behind the BFF, never in the browser
+
+The Angular app only ever sees the BFF's decision DTOs (BFF-lite, ADR-0001). All ZGW
+awkwardness — URL-as-identity, cross-service joins, JWT auth, pagination — is absorbed by the
+.NET BFF. Flipping the data source from local SQLite to OpenZaak is a **backend config change**
+with **zero frontend change and no api-client drift**.
+
+## The seam (data source by config)
+
+- `Data/IZaakSource.cs` — the cases READ interface. Returns the existing
+  `ApplicationSummaryDto`, so each implementation owns its own mapping.
+- `Data/LocalZaakSource.cs` — **default**; reads the local SQLite `ApplicationStore`
+  (offline, unchanged behaviour).
+- `Zgw/OpenZaakZaakSource.cs` — the OpenZaak client; selected only when `Zgw:Enabled=true`.
+- Wiring (`Program.cs`): `if (Zgw:Enabled) AddHttpClient<IZaakSource, OpenZaakZaakSource>()
+else AddSingleton<IZaakSource, LocalZaakSource>()`. The `/admin/cases` endpoint resolves
+  `IZaakSource` from DI — routes + DTOs untouched.
+
+## The ZGW client (`backend/src/BigRegister.Api/Zgw/`)
+
+- `ZgwOptions.cs` — bound from the `Zgw` appsettings section: `Enabled`, per-service base URLs
+  (`ZrcBaseUrl`, `ZtcBaseUrl`), `ClientId`, `Secret`, `UserId`, `UserRepresentation`. The five
+  ZGW APIs are separate base URLs; slice 1 needs only Zaken (ZRC) + Catalogi (ZTC).
+- `ZgwTokenProvider.cs` — mints an **HS256 JWT per call** (`iss`/`client_id`/`iat`/`user_id`/
+  `user_representation`). No refresh flow — OpenZaak expires tokens 1h past `iat`, so per-call
+  minting is the recommended pattern. Hand-rolled (no `Microsoft.IdentityModel.*` dependency).
+- `ZgwZaakMapper.cs` — the anti-corruption map: ZGW Zaak → `ApplicationSummaryDto`. This is
+  where **URL identity** becomes the trailing uuid and the **zaaktype URL** is resolved to a
+  human label (the cross-service join).
+- `OpenZaakZaakSource.cs` — follows `{count,next,previous,results}` pagination, resolves +
+  caches zaaktype labels, attaches `Authorization: Bearer <jwt>`.
+
+## The five ZGW APIs (context for later slices)
+
+| API          | Component | Used by                                             |
+| ------------ | --------- | --------------------------------------------------- |
+| Zaken        | ZRC       | slice 1 (read), WP-50 (create)                      |
+| Catalogi     | ZTC       | slice 1 (zaaktype label; also type URLs for create) |
+| Documenten   | DRC       | WP-51 (upload + zaak↔document link)                 |
+| Besluiten    | BRC       | later (formal decisions)                            |
+| Notificaties | NRC       | WP-52 (live status via webhooks, not polling)       |
+
+## How to add the next slice
+
+1. **Read** — extend `IZaakSource` (or add a sibling interface, e.g. `IDocumentSource`) with
+   the new operation; implement it on both `LocalZaakSource` and the OpenZaak source. Keep the
+   return type the existing DTO so the FE never changes.
+2. **Write** (create-zaak, WP-50) — a create needs a `zaaktype` URL from Catalogi (OpenZaak
+   validates it by fetching), then usually a follow-up `status` + `rol`. Route it through the
+   existing submit/mutation seam.
+3. **Enforce server-side** for anything the FE gates — a config value the FE echoes is never
+   the authority (ADR-0001).
+
+## Coupling
+
+Low and one-directional. Consumer coupling is near zero — `IZaakSource` is injected at one
+endpoint, and the FE is fully decoupled by the DTO. The producer side is contained in `Zgw/`:
+add a slice by adding a source method + a mapper case, not by touching the FE or the contract.
+Watch the **sync-over-async** `ponytail:` note in `OpenZaakZaakSource` — make the cases read
+path async if OpenZaak becomes the default.
+
+## Config
+
+```jsonc
+// appsettings.json — off by default (POC runs offline on the local store)
+"Zgw": {
+  "Enabled": true,
+  "ZrcBaseUrl": "https://open-zaak.example/zaken/api/v1",
+  "ZtcBaseUrl": "https://open-zaak.example/catalogi/api/v1",
+  "ClientId": "big-register", "Secret": "<from a secret store>",
+  "UserId": "<session user>", "UserRepresentation": "<session name>"
+}
+```
+
+## See also
+
+- [ADR-0005 — OpenZaak behind the BFF](architecture/0005-openzaak-behind-bff.md) — the decision.
+- [ADR-0001 — BFF-lite + decision DTOs](architecture/0001-bff-lite-decision-dtos.md) — why the FE doesn't change.
+- [WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (this), WP-50/51/52 (later slices).
+- `backend/src/BigRegister.Api/Zgw/` — the client; `Data/IZaakSource.cs` — the seam.
+- [ZGW standard (VNG)](https://vng-realisatie.github.io/gemma-zaken/) · [OpenZaak auth docs](https://open-zaak.readthedocs.io/en/stable/client-development/authentication.html).

docs/reference/stamdata.md

@@ -6,6 +6,11 @@ build**, not a runtime-editable database. For the _why_, see
 [ADR-0004 — Stamdata as code](architecture/0004-stamdata-as-code.md); this page is _how the
 code is laid out and how to add a table without coupling_. Built in WP-29, hardened in WP-48.
 
+Tables today: `professions` (opleiding-program → beroep), `beroepen` (the BIG professions master
+list), `opleidingen` (temporal; `beroep` → `beroepen.code`) and `specialismen` (`beroep` →
+`beroepen.code`). The last two are **stamdata → stamdata** references — one table keyed on by two
+others — enforced by the CI gate below.
+
 ## The one rule that shapes everything: no runtime write path
 
 The catalog is the source of truth and lives in code. The admin editor **downloads** an
@@ -50,8 +55,11 @@ component**. This is the payoff of the schema-driven design.
 
 `backend/tests/BigRegister.Tests/StamdataValidationTests.cs`. `Every_catalog_table_is_valid`
 covers every registered table generically; the `StamdataRef` list catches dangling
-references (today: `Diploma.Opleiding → professions.program`). A bad edit, an orphaning
-delete, or a premature expire **fails the PR build** — never prod.
+references — both seed → stamdata (`Diploma.Opleiding → professions.program`) and
+stamdata → stamdata (`Opleiding.beroep → beroepen.code`, `Specialisme.beroep →
+beroepen.code`). A bad edit, an orphaning delete, or a premature expire **fails the PR
+build** — never prod. Adding a cross-table FK is one `StamdataRef` entry: the referencing
+keys + a resolver against the target table's (valid-today) keys.
 
 ## Coupling