feat(seo): add /llms.txt and markdown versions of blog posts and legal pages

[datlechin]

• /llms.txt (llmstxt.org), English, built from the page registry: docs llms.txt, product, databases, comparisons, guides, then release posts and legal pages under Optional. Titles and descriptions come from the content files.
• Blog posts and legal pages answer at {page}.md in every locale they render in, with the date, correction, last-updated line and translation notice the page shows. {#id} and controls are dropped; an image slot becomes the image production shows, or nothing.
• .md responses send Content-Type: text/markdown, Link: <page>; rel="canonical" and the edge cache header.
• Page head links the markdown version with rel="alternate" type="text/markdown" (seo.markdown prop).
• The {page}.md route name resolves to its page in the registry, so the page middleware gates it.
• No Accept: text/markdown negotiation (Cloudflare caches HTML and ignores Vary) and no AI preference line in robots.txt (Content-Signal is Cloudflare's, aipref Content-Usage is still a draft). Both noted in docs/architecture.md.