Remove Codecov integration from CI

[swolg]

This pull request primarily removes the Codecov integration and related test metrics from both the backend and frontend CI workflows. It also updates documentation to reflect these changes and clarifies how coverage is enforced. The most important changes are:

CI Workflow Updates:

• Removed steps for extracting test metrics and uploading backend coverage reports to Codecov from .github/workflows/quality.yml.
• Kept frontend Codecov upload commented out, maintaining the current state of not uploading frontend coverage.

Documentation Updates:

• Removed the Codecov test coverage badge from the README.md.
• Updated the coverage note in docs/guides/testing.md to indicate that the 75%+ coverage threshold is enforced by CI, rather than reporting the current percentage.