CI: install pytest in backend test workflow

[Copilot]

The backend-ci workflow’s test job failed because it invoked python -m pytest tests -q without installing pytest. This PR updates the workflow so the test runner is present in the job environment.

• Root cause

  • backend/requirements.txt contains runtime deps only; pytest was missing from CI environment while still required by the workflow command.

• Workflow change

  • Updated .github/workflows/backend-ci.yml to install pytest in the dependency setup step before running tests.

• Behavioral impact

  • Aligns CI environment with workflow expectations for the test job.
  • No application/runtime code paths were modified.

- name: Install dependencies
  run: |
    python -m pip install --upgrade pip
    pip install -r requirements.txt
    pip install pytest