Revert "Remove redundant OpenAPI CI job"

This reverts commit 73e061c8.

.gitlab-ci.yml

@@ -59,6 +59,17 @@ backend-migrations:
     - pip install -e .
     - arkindex/manage.py makemigrations --check --noinput --dry-run -v 3
 
+backend-openapi:
+  extends: .backend-setup
+  stage: build
+
+  script:
+    - ci/openapi.sh
+
+  artifacts:
+    paths:
+      - output/
+
 backend-static:
   extends: .backend-setup
   stage: build