Add GitHub Actions CI Workflow

This PR introduces continuous integration using GitHub Actions to automatically test the project across multiple Python versions.

Changes

• New CI workflow (.github/workflows/ci.yml):
  • Runs on pushes to main and on all pull requests
  • Tests against Python 3.9 and 3.14 (minimum and maximum supported versions)
  • Installs project with dev dependencies
  • Runs pytest with coverage reporting

Benefits

• Ensures code compatibility across Python versions
• Catches issues early before merging
• Provides test coverage insights
• Automates the testing process for all contributors

Testing

The workflow has been validated locally with:

pytest --cov=addok_fr --cov-report=term-missing

All tests pass successfully with the current codebase.