A cached image was served until eviction, so a change at the same URL
was never picked up.
After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it, and older variants are converted
again. A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.
Variant rows keep the source's ETag and Last-Modified, so a variant
whose source was evicted is checked the same way.
A variant file's modification time is its source's check time when it
was made or renewed, so a hit reads no database row.
Model: opus-5-5
