fix(infra): cap celery workers at 2 so the shared node stops OOM-killing CI (refs #182)

Unset CELERY_WORKER_CONCURRENCY makes oz-celery and nrc-celery fork one
process per CPU — 22 each on the lab node, ~225 MB apiece — and Talos'
OOM controller was killing the runner mid-verify-stack. Same lever as the
uWSGI caps (#144/#145/#147); objecten-celery already defaults to 1.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

infra/helm/big-reference/values.yaml

@@ -76,6 +76,7 @@ envGroups:
   oz:
     UWSGI_PROCESSES: "1"
     UWSGI_THREADS: "2"
+    CELERY_WORKER_CONCURRENCY: "2"
     DJANGO_SETTINGS_MODULE: openzaak.conf.docker
     SECRET_KEY: dev-only-not-for-production
     DB_HOST: oz-db
@@ -98,6 +99,7 @@ envGroups:
   nrc:
     UWSGI_PROCESSES: "1"
     UWSGI_THREADS: "2"
+    CELERY_WORKER_CONCURRENCY: "2"
     DJANGO_SETTINGS_MODULE: nrc.conf.docker
     SECRET_KEY: dev-only-not-for-production
     DB_HOST: nrc-db