fix: Move md tracking to middleware for static optimization (#11535)
### Description
Move markdown tracking from route handlers to middleware to preserve
static optimization. Reading request headers in route handlers de-opts
them into dynamic mode.
Changes:
- Add middleware.ts to track llms.txt, llms.md/*, and llms-full.txt
- Remove tracking code from llms.txt and llms.md route handlers
- Use event.waitUntil() for fire-and-forget tracking
### Testing Instructions
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Anthony Shew <anthonyshew@gmail.com>
Co-authored-by: Anthony Shew <anthony.shew@vercel.com>