fix(worker): unbreak wiki AI with free provider chain

- Fix ReferenceError in /api/chat: openRouterRes was block-scoped and
  referenced outside its block, crashing every request with a 500
- Replace single OpenRouter dependency with a free provider chain:
  Workers AI (env.AI binding) → Groq → GitHub Models → OpenRouter free
- Add R2 cache for generated guides (30-day TTL) so repeated requests
  cost zero neurons / tokens
- Add 30s timeouts to all provider fetches
- Rewrite AI health check to exercise the real generateGuide path
  (previously it pinged OpenRouter directly and missed worker bugs)
- Route /api/news/ai through the same chain with proper validation

Author: spivanatalie64