feat: Add pytest coverage reporting to CI

[db-tycoon-stephen]

Problem

No test coverage tracking or reporting. Can't measure progress toward 80% coverage target.

Proposed Solution

1. Add pytest --cov=src --cov=scripts --cov-report=xml to CI test step
2. Upload coverage to Codecov (or similar)
3. Add coverage badge to README
4. Set minimum coverage threshold (70% initially)

Acceptance Criteria

• CI generates coverage report on every run
• Coverage badge visible in README
• Coverage trend trackable over time