Add test coverage threshold enforcement to CI

[caitlon]

Problem Statement

The project has high test coverage for core library, but CI does not enforce a minimum coverage threshold. Coverage could regress without failing the build.

Current State

• pytest with coverage runs in CI
• No minimum threshold configured
• Coverage report generated but not enforced
• No coverage badge in README

Proposed Solution

1. Add coverage threshold to pytest configuration
2. Fail CI if coverage drops below threshold
3. Add coverage badge to README (using codecov or similar)

Acceptance Criteria

• pytest coverage threshold set (suggest 90% minimum for core, 80% overall)
• CI fails if coverage drops below threshold
• Coverage report uploaded as artifact or to codecov
• Coverage badge added to README
• Documentation on how to check coverage locally

Technical Notes

• Add --cov-fail-under=90 to pytest command
• Consider separate thresholds for src/ (core) and api/ (web)
• Codecov integration available via GitHub Action