Add Lighthouse CI performance checks

[haljac]

Problem

There are no automated performance, accessibility, or SEO checks. Regressions in page load speed, a11y compliance, or SEO metadata go undetected until users report them.

Scope of Work

Add Lighthouse CI to the CI pipeline

Use treosh/lighthouse-ci-action to run Lighthouse against the built dist/ directory.

Setup:

1. After the npm run build step, serve dist/ locally (e.g., with npx serve dist -l 8080 &)
2. Run Lighthouse CI against http://localhost:8080
3. Configure thresholds in a lighthouserc.json or inline

Suggested initial thresholds (lenient, ratchet up over time):

{
  "ci": {
    "assert": {
      "assertions": {
        "categories:performance": ["warn", { "minScore": 0.8 }],
        "categories:accessibility": ["error", { "minScore": 0.9 }],
        "categories:best-practices": ["warn", { "minScore": 0.8 }],
        "categories:seo": ["warn", { "minScore": 0.9 }]
      }
    }
  }
}

Pages to test: At minimum, / (home) and /blog (listing).

Acceptance Criteria

• Lighthouse CI runs in the CI pipeline after build
• Performance, accessibility, best practices, and SEO scores are checked
• Accessibility failures (error level) block the build
• Performance/SEO failures are warnings (don't block but are visible)
• Lighthouse report is uploaded as a CI artifact
• Thresholds are documented and easy to update