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/README.md

@@ -93,6 +93,10 @@ for its existing violations, so every WP ends green.
 | [WP-46](WP-46-vitest-coverage.md)              | Vitest coverage (report + report-only thresholds)                                  | 8 · platform/DX/showcase    | done   |
 | [WP-47](WP-47-feature-flags.md)                | Runtime feature flags (catalog-in-code, admin toggle, FE+backend)                  | 8 · platform/DX/showcase    | done   |
 | [WP-48](WP-48-stamdata-deletion-protection.md) | Stamdata deletion protection (CI referential gate + editor expire/warn)            | 8 · platform/DX/showcase    | done   |
+| [WP-49](WP-49-openzaak-zaken-read-seam.md)     | OpenZaak zaken read seam (IZaakSource + ZGW client, config-gated, offline default) | 9 · OpenZaak/ZGW            | done   |
+| [WP-50](WP-50-openzaak-create-zaak.md)         | OpenZaak create-zaak (first write slice)                                           | 9 · OpenZaak/ZGW            | todo   |
+| [WP-51](WP-51-openzaak-documenten.md)          | OpenZaak Documenten (DRC) upload + zaak link                                       | 9 · OpenZaak/ZGW            | todo   |
+| [WP-52](WP-52-openzaak-notificaties.md)        | OpenZaak Notificaties (NRC) live status via webhook                                | 9 · OpenZaak/ZGW            | todo   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);
 03/04 before 05–09 (boundaries stop new violations during refactors); 06 before 07 (typed
@@ -114,6 +118,10 @@ WP-40 (P2) → WP-43 (P3) → WP-41 → WP-42 → WP-44 → WP-45 (P4). Ordering
 make the generator simple) + 43; 45 (`create-ssp`) needs 43+44. 37/38/39/40/43/46 are otherwise
 independent. Two open tool forks, decided as step 1 of their WP: 38 dependency-cruiser vs Sheriff;
 43 plop vs Angular schematics.
+Phase 9 (OpenZaak/ZGW integration, WP-49..52) is strictly ordered 49 → 50 → 51 → 52: 49 lands
+the source seam + ZGW client the rest reuse; 50 (create) needs a zaak to exist; 51 (documents)
+links to 50's zaak; 52 (notificaties) reacts to changes on the zaken 49–51 manage. All slices
+stay behind `Zgw:Enabled` (default off) so the POC keeps running offline.
 
 ## WP template