docs: explain how to check OpenZaak is running

docker compose up at the repo root does not start OpenZaak. A new reader
could easily assume it does. Add a short section to
backend/openzaak/README.md that shows how to check the four containers and
how to curl OpenZaak directly, using the same probe
bootstrap-catalogus.sh already relies on. Add one line to the root README
pointing there.

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

README.md

@@ -32,6 +32,9 @@ docker compose up
 # API + Swagger        → http://localhost:5000/swagger
 ```
 
+This does not start OpenZaak. The BFF uses a local, in-memory case store instead. To run
+against a real OpenZaak, see [backend/openzaak/README.md](backend/openzaak/README.md).
+
 Or run the pieces yourself:
 
 ```bash

backend/openzaak/README.md

@@ -34,6 +34,32 @@ available if you want a truly clean slate:
 docker compose -f docker-compose.openzaak.yml down -v && docker compose -f docker-compose.openzaak.yml up -d
 ```
 
+## Check it is running
+
+**`docker compose up` at the repo root does not start OpenZaak.** OpenZaak is a separate
+stack. Bring it up with the command above, or with `scripts/openzaak-ui-up.sh` below.
+
+Check the containers:
+
+```bash
+docker compose -f docker-compose.openzaak.yml ps
+```
+
+Look for four containers: `db`, `redis`, `web-init`, `web`. `db` and `redis` show a health
+status. `web-init` runs once and exits. Its exit code must be `0`. `web` has no health
+status. Check it directly instead:
+
+```bash
+curl -sS -o /dev/null -w '%{http_code}\n' http://localhost:8000/catalogi/api/v1/catalogussen
+```
+
+A `200`, `401`, or `403` response means OpenZaak is up and answering. This is the same
+check `bootstrap-catalogus.sh` uses to wait for OpenZaak before it seeds anything.
+
+To confirm the BFF (not just OpenZaak) is wired up and can write to it, see "Testing the
+Angular UI against this harness" below — `scripts/openzaak-ui-up.sh` submits a real test
+case and confirms OpenZaak received it.
+
 This content has no `setup_configuration` (declarative-YAML) equivalent: reading the
 `django_setup_configuration` steps installed inside the `openzaak/open-zaak:1.29.1` image
 itself confirms the only app-registered steps are sites/credentials/applicaties (already used