Add sitemap.ts and robots.ts

[AnayDhawan]

Problem

The app has no sitemap and no robots route, so search engines have to discover every page by following links from the homepage.

Where to start

• src/app/ - add sitemap.ts and robots.ts here
• Next.js generates both from these files automatically, no extra config needed
• Next.js sitemap docs

What to build

sitemap.ts returning the static routes (/, /map, /calendar, /about, /contribute, /docs and its subpages, /legal/*).

robots.ts allowing crawling and pointing at the sitemap URL.

If the per-city pages from #127 have already landed, include those too. If not, just do the static routes and say so in your PR; the city pages can be added afterwards.

Acceptance criteria

• /sitemap.xml returns valid XML listing the static routes
• /robots.txt allows crawling and references the sitemap
• No route appears twice
• Nothing under /auth or /login is listed

Verifying

npm install
npm run build
npm run start
curl http://localhost:3000/sitemap.xml
curl http://localhost:3000/robots.txt

Scope: small, under an hour. Good first issue.

New to the repo? Start with CONTRIBUTING.md.