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/project/backlog/WP-51-openzaak-documenten.md

@@ -0,0 +1,52 @@
+# WP-51 — OpenZaak Documenten (DRC) upload + link
+
+Status: todo
+Phase: 9 — OpenZaak / ZGW integration
+
+## Why
+
+Uploaded documents currently persist as bytes in local SQLite (`DocumentStore`). To be
+production-ready they must live in OpenZaak's **Documenten API (DRC)** as
+`enkelvoudiginformatieobjecten`, linked to a Zaak via `zaakinformatieobject` — behind the same
+config gate, still without a FE contract change.
+
+## Read first
+
+- [openzaak-integration.md](../reference/openzaak-integration.md)
+- WP-49 (the seam pattern), WP-50 (create-zaak, the zaak to link to)
+
+## Decisions (pre-made, don't relitigate)
+
+- Introduce an `IDocumentSource` sibling of `IZaakSource`; local impl = `DocumentStore`,
+  OpenZaak impl = DRC. FE upload DTOs unchanged.
+- A document needs an `informatieobjecttype` **URL** from Catalogi (like zaaktype for a zaak).
+- Upload → returns document URL → `zaakinformatieobject` links it to the zaak URL.
+
+## Files
+
+- `Data/IDocumentSource.cs`, `Data/LocalDocumentSource.cs`, `Zgw/OpenZaakDocumentSource.cs`
+- `ZgwOptions`: `informatieobjecttype` URL(s) + `DrcBaseUrl`.
+
+## Steps
+
+1. Abstract the upload/read/link paths behind `IDocumentSource`.
+2. OpenZaak impl: POST `enkelvoudiginformatieobjecten` (content), then POST `zaakinformatieobjecten`.
+3. Map DRC document metadata back into the existing document DTOs.
+
+## Acceptance criteria
+
+- [ ] With `Zgw:Enabled=true`, an upload lands in DRC and is linked to its zaak.
+- [ ] FE upload/list DTOs unchanged; no api-client drift.
+- [ ] Tests cover the DRC POST bodies + the zaak-link step (stub handler).
+
+## Verification
+
+`dotnet test`; against a docker OpenZaak if available.
+
+## Out of scope
+
+Notificaties (WP-52), content virus-scanning / blob-storage tuning.
+
+## Risks
+
+Large file content over base64/multipart — mind memory; stream if needed.