feat(zgw): docker OpenZaak integration-test harness (WP-54)

Opt-in docker-compose (postgres+redis+OpenZaak, no celery/nginx) +
bootstrap-catalogus.sh seed a real OpenZaak instance; OpenZaakIntegrationTests
(Category=Integration, excluded from default dotnet test/CI) proves the ZGW
seam against it for the first time. That live run caught a real bug:
ZgwHttpClient never sent Content-Crs/Accept-Crs headers, so every write would
412 against a spec-compliant OpenZaak — fixed alongside the harness.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -51,3 +51,6 @@ storybook-static
 /playwright-report
 /blob-report
 /playwright/.cache
+
+# WP-54: bootstrap-catalogus.sh's own record of what it seeded into a local OpenZaak run
+backend/openzaak/seeded.env