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 ensuring pytest was installed in the runner environment. This PR adds the missing test dependency in the workflow setup path.

• Failure mode

  • test job hit No module named pytest during the test execution step.

• Workflow change

  • Updated .github/workflows/backend-ci.yml to install pytest in the existing dependency install step before running the test command.
  • Scope is intentionally limited to CI wiring; no runtime/backend application code changes.

• Relevant diff

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