[CI] Add Lighthouse CI budgets on preview deployments

[andrewy1n]

Parent epic: #115

Summary

Add Lighthouse CI checks against preview deployments with explicit quality budgets for performance and core UX metrics.

Scope

• Add Lighthouse CI configuration for preview deployment URLs.
• Define initial thresholds for key scores/metrics (Performance, Accessibility, Best Practices, SEO as agreed).
• Integrate Lighthouse run into CI workflow and PR checks.
• Upload Lighthouse reports/artifacts for regressions.

Acceptance Criteria

• Lighthouse CI runs against preview URL in pull requests (or defined branch policy).
• Thresholds are codified and enforced in CI.
• Lighthouse report artifacts are accessible on failure.
• Documentation explains local and CI usage, including thresholds and tuning.

Dependencies

• Preview URL e2e/deploy wiring should be available first (see sibling issue).