[M3][Epic 3.1] Replace placeholder CI with real tests, lint, and coverage

[POWDER-RANGER]

Problem

Current .github/workflows/ci.yml is a placeholder. Build production CI pipeline.

Tasks

• Backend unit tests (Jest/Mocha)
• Frontend unit tests (Jest + React Testing Library)
• ML tests (pytest with fixtures)
• Add ESLint + Prettier for backend/frontend
• Add Black + Flake8 for ML service
• Add TypeScript type checking (tsc --noEmit)
• Configure Codecov upload
• Set coverage threshold (60% initially, target 80%+)
• Add branch protection rules (require passing CI)
• Update README badge to point to actual workflow

Acceptance Criteria

• ✅ PRs cannot merge unless tests + lint pass
• ✅ Coverage threshold enforced in CI
• ✅ Codecov badge in README displays actual coverage
• ✅ CI runs complete in <5 minutes

Priority: P0 - Blocker for M3
Labels: ci/cd, testing, M3, P0
Related: Addresses issue #4 - Missing CI/CD Pipeline