fix(ci): add SECRET_KEY to backend workflow

[qiuethan]

Summary

This PR fixes the backend CI pipeline by injecting the SECRET_KEY environment variable into the migration and test steps. Django requires this key to start, and its absence was causing the pipeline to fail.

Changes

• Updated .github/workflows/backend-ci.yml:
  • Added SECRET_KEY: ${{ secrets.SECRET_KEY }} to the "Run migrations" step.
  • Added SECRET_KEY: ${{ secrets.SECRET_KEY }} to the "Run tests with pytest" step.

How to Test

1. Push these changes to a branch.
2. Navigate to the "Actions" tab in the GitHub repository.
3. Verify that the "Backend CI" workflow runs and completes the "Run migrations" and "Run tests with pytest" steps successfully.

Checklist

• Tests added or updated (CI configuration updated)
• CI passes (lint, tests, build)

[vivjd]

LGTM! thankssss