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>

scripts/ci-local.sh

@@ -18,7 +18,7 @@ step "check:tokens";              npm run check:tokens
 step "test (vitest + coverage)";  npm run test:coverage
 step "build --localize (nl+en)";  npx ng build --localize
 step "npm audit (shipped deps)";  npm audit --omit=dev
-step "backend format + tests";    ( cd backend && dotnet format BigRegister.slnx --verify-no-changes && dotnet test BigRegister.slnx )
+step "backend format + tests";    ( cd backend && dotnet format BigRegister.slnx --verify-no-changes && dotnet test BigRegister.slnx --filter "Category!=Integration" )
 step "showcase snippets drift";   npm run gen:snippets && git diff --exit-code src/app/showcase/snippets.generated.ts
 step "api-client drift";          npm run gen:api && git diff --exit-code src/app/shared/infrastructure/api-client.ts backend/swagger.json