Add coverage measurement and a coverage gate to CI

[lucastononro]

Problem. backend-test in .github/workflows/ci.yml runs pytest tests/ -v --tb=short with no coverage; pytest-cov isn't even in backend/requirements.txt. There's no visibility into what the 35 test files actually exercise.

Why it matters. Regressions in untested modules (runner, sandbox) pass green.

Suggested fix. Add pytest-cov, run pytest --cov=. --cov-report=xml --cov-report=term-missing, upload to Codecov/artifact, and set a --cov-fail-under floor.

[lucastononro]

Addressed by #149, now merged into integration branch staging-v0.0.5 (ledger: STAGING-v0.0.5.md). Will land on main via the staging merge — closing.