@cache: send an Age header on hits so downstream freshness does not reach 2×ttl

[allen0099]

A hit sends the full max-age=ttl and no Age. An entry stored at t=0 with ttl=60 and served at t=59 is fresh downstream until t=119, so after invalidate() a browser or CDN can keep serving the old response for almost twice the ttl (RFC 9111 §4.2.3, §5.1).

Proposal

Store stored_at in CacheEntry (optional, like status_code) and emit Age on hits and cached-ETag 304s, or lower max-age to the remaining ttl.

[allen0099]

Seen again while building a new app from the docs: a hit 3 s after the miss still sent Cache-Control: max-age=60, and there was no Age header, so a browser or CDN can hold the content for up to about 2×ttl. Moving this to 0.3.9 together with the other issues from that exercise.