#75 fix(llms): send a public max-age so shared caches can store llms.txt

Live check on glyphs.zoolutions.llc/llms.txt showed Cache-Control: public with no max-age, so the newly enabled proxy.cache never stored it. expires_in 300, public: true before stale? fixes that; the etag revalidation is unchanged.

https://claude.ai/code/session_01SdEc37DFCkYzwpQpadu9My

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Summary by cubic

Fixes /llms.txt caching so shared caches like dash-proxy and CDNs can store it. Previously Cache-Control: public had no max-age, so RFC 9111-compliant caches skipped the response. Added expires_in 300, public: true before the etag stale check; revalidation still works.