A cached source and its variants were served until eviction, so a
changed image at the same URL was never picked up.
Once a source was last checked 24 hours ago, the next request for one
of its images asks upstream with If-None-Match and If-Modified-Since,
from the ETag and Last-Modified it stored. A 304 renews the source and
its variant without a download or a conversion. New bytes replace the
source, and each variant made before them is converted again. A failed
check serves the cached image and puts the URL in the negative cache.
Sources and variants not checked successfully for 30 days are evicted.
A variant file's modification time is its source's check time, so a
hit still makes no database call.
Existing tests change only in call signatures. The test that asserted
two-day-old entries are served without asking upstream is replaced by
the new ones.
Model: opus-5-5
