feat: add SEO metadata and structured data

[VitorBernalRodrigues]

Summary

Adds the SEO metadata infrastructure requested in #66.

• Adds generated sitemap.xml using the shared site map data.
• Adds robots.txt with production and preview-specific indexing behavior.
• Blocks Vercel preview deployments from being indexed.
• Adds Organization and SoftwareApplication JSON-LD structured data.
• Adds route-specific descriptions for all public pages.
• Extracts the public route definitions into a shared module used by both the site map UI and metadata routes.
• Adds focused SEO tests without introducing new dependencies.

Implementation

The public route list is now shared through lib/site-map.ts, avoiding a duplicated route inventory between the UI and sitemap generation.

Canonical URLs are resolved from:

1. VERCEL_PROJECT_PRODUCTION_URL
2. VERCEL_URL
3. http://localhost:3000 for local development

Preview deployments are detected through VERCEL_ENV === "preview" and return:

• Disallow: / from robots.txt
• noindex, nofollow HTML metadata

Production and local development remain indexable.

Structured data is rendered server-side and includes:

• Organization
• SoftwareApplication

The JSON-LD serialization escapes < to avoid prematurely terminating the script element.

Testing

Passed locally:

• npm run test:seo — 6/6 tests
• npm run lint
• npx tsc --noEmit
• npm run build
• git diff --check

The production build generated both /robots.txt and /sitemap.xml.

The generated sitemap, robots metadata, structured data, and route metadata were also verified with a local production server.

Notes

No new dependencies were added and there are no visual changes.

Closes #66