Add URL-based pagination for crawler accessibility

- Add /api/comments/[author]/[permlink] API route with in-memory caching
- Add visible pagination links that crawlers can follow without JS
- Support ?comments_page=N query parameter for URL-based navigation
- Pass initialPage to CommentList for SSR of specific pages
- Add pagination translation keys for all locales

The pagination links appear below the "Show more" button, providing both:
- JS-enabled "Show more" for smooth UX
- Real <a href> links for crawlers to discover all comments

Addresses issue #720 crawler accessibility concerns.