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/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).