Add CI to run backend + frontend tests on PRs

[michaelzwang13]

The only GitHub workflows are the Claude review ones. Nothing runs the test suites on a PR, so regressions can merge unnoticed.

Scope

A GitHub Actions workflow, triggered on PRs to main, that runs:

• Backend: pytest (100 tests). Note Apple-Silicon parity isn't needed on CI Linux runners — a standard python -m pytest is fine.
• Frontend: bun run test (Vitest, 14 tests), bun run lint, bun run build.

Block merge on failure.