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>

.github/workflows/ci.yml

@@ -92,7 +92,9 @@ jobs:
           key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
           restore-keys: nuget-${{ runner.os }}-
       - run: dotnet format backend/BigRegister.slnx --verify-no-changes
-      - run: dotnet test backend/BigRegister.slnx
+      # Category=Integration (WP-54, OpenZaakIntegrationTests) needs a live OpenZaak — opt-in,
+      # run manually against backend/openzaak/ (see its README), never in CI.
+      - run: dotnet test backend/BigRegister.slnx --filter "Category!=Integration"
 
   e2e:
     # Smoke-level Playwright run against the REAL FE+backend (WP-19) — a fresh