Cached images were served until eviction, so upstream changes were
missed.
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; 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 for checking a
variant of an evicted source.
A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.
Model: opus-5-5
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