CI: add coverage threshold once codebase stabilises

[koinsaari]

Once the project is past early-stage churn, add a minimum coverage threshold to the unit test job to fail the build on regressions.

Suggested approach

• Add -coverprofile=coverage.out is already in place (added in CI: run tests with -race and report coverage #19)
• Use go tool cover -func=coverage.out output to compute total, or use a dedicated action
• Agree on a threshold (e.g. 70–80%) and enforce with something like:

  go tool cover -func=coverage.out | grep total | awk '{if ($3+0 < 70) exit 1}'

• Alternatively integrate Codecov for trend tracking and PR comments

When to do this

After the core API endpoints and accessibility engine have stable test coverage — not mid-feature when coverage naturally dips.

Files: .github/workflows/ci.yml