The ETag was the pixel hash of the original image, so a CoverArtQuality or
EnableWebPEncoding change altered the resized bytes without changing the ETag —
revalidating clients got a spurious 304 and kept the old encoding. Resized responses
now carry a representation ETag (hash + size + square + encode settings) used for the
ETag header and If-None-Match, while the immutable decision stays on the pixel hash
(URLs remain pixel-identity per the spec, so hash-suffixed clients keep zero-request
caching). Full-size originals fall back to the pixel hash as before.
