Lighthouse budget in CI #34

Why

The site should stay fast on a phone as it grows.

In this repo today

The app has two page types, / and /project/<id>, and it needs the API to render anything.

What to build

• A CI job that builds the web app, serves it with the API on the sample dataset (see the local dev issue), and runs Lighthouse CI against / and one project page.
• The job fails if the mobile performance score is below 90.
• The report is uploaded as a workflow artifact.

Acceptance criteria

• The job runs on every pull request and uploads the report.
• It fails when the score drops below 90.

See CONTRIBUTING.md for conventions.