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>

backend/src/BigRegister.Api/Zgw/ZgwHttpClient.cs

@@ -38,5 +38,11 @@ internal sealed class ZgwHttpClient(HttpClient http, ZgwTokenProvider tokens)
   {
     req.Headers.Authorization = new AuthenticationHeaderValue("Bearer", caller is null ? tokens.Mint() : tokens.Mint(caller));
     req.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
+    // Every ZGW request must declare a coordinate reference system, even when no geometry is
+    // involved (Zaak has an optional zaakgeometrie) — a real OpenZaak 412s ("Content-Crs
+    // header ontbreekt") without it. Only surfaced by WP-54's live harness: the fixture/stub
+    // tests never modelled this header, so this bug shipped unnoticed since WP-49/50.
+    req.Headers.Add("Accept-Crs", "EPSG:4326");
+    if (req.Content is not null) req.Content.Headers.Add("Content-Crs", "EPSG:4326");
   }
 }