Seo and llm endpoints

[driveton]

Summary

• Add <meta name="description"> tags to book and leafable show views, improving search engine snippets and social preview cards
• Expand the LLM-friendly text endpoint (/llms.txt) to include a detailed per-book table of contents with direct links to each page, making the full content hierarchy discoverable by LLMs and crawlers
• Fix the LLM endpoint query to use accessable_or_published (instead of published only) and eager load leaves: :leafable to prevent N+1s
• Fix malformed og:image/twitter:image URLs where root_url was being prepended to url_for(attachment), which already returns an absolute URL, resulting in a double-slash (e.g. https://example.com//rails/active_storage/...)