[Task]: Add CI workflow to run tests and report code coverage

[dellch]

Description

Add a GitHub Actions workflow that runs on pull requests and executes the test suite with code coverage reporting. The workflow should:

• Run make test (which executes all test suites)
• Collect code coverage via coverlet (backend) and any frontend coverage tooling when available
• Report coverage results (e.g., as a PR comment or via a coverage service)
• Fail the PR if tests fail

Dependencies

Depends on #3 (frontend scaffold) and #7 (frontend test infrastructure) for frontend coverage.
Backend tests can run independently once #4 is merged.