[GEO][P1] Generate llms.txt and llms-full.txt dynamically from Payload #101

Problem

public/llms.txt is static and lists no articles, so LLM agents reading it cannot discover recent analysis.

Proposal

• Replace the static file with a cached route /llms.txt that keeps the current editorial header and appends the latest published FR/EN posts (title, canonical URL, excerpt).
• Add /llms-full.txt with the plain-text body of recent published posts (bounded size), each prefixed by title, URL, date and sources.
• Only advertise URLs that the public article route actually serves (same rules as the posts sitemap).
• Invalidate with the existing post revalidation tags + bounded TTL.

Acceptance

• Both routes return text/plain; charset=utf-8, 200, and list only published, fully localized posts.
• Unit tests for the formatting helpers.