- Add contentRoutes handler factory in routes/content.ts
- GET /pages/:id/screenshot returns PNG as base64
- GET /pages/:id/text returns document.body.innerText
- GET /pages/:id/html returns document.documentElement.outerHTML
- All use safeStringify, 404 on PAGE_NOT_FOUND
- Register contentRoutes in server.ts
- Add 8 integration tests with mock BrowserManager
