Hi! I'd like to help improve the CI pipeline. Two small improvements:

1. Dependency caching — Add actions/cache for Poetry's virtualenv so dependencies aren't reinstalled on every run. Should cut CI time noticeably.
2. Code coverage reporting — Add pytest-cov and upload results to Codecov. This gives visibility into test coverage gaps without changing any existing tests.

Both are non-breaking changes to the workflow files only. Happy to split into separate PRs if preferred. Would this be welcome?