Pillow 11.0.x did not include native AVIF support — that was added in
11.1.0. The ~=11.0.0 pin restricted upgrades to 11.0.x, so the
libavif-dev system package installed in the Dockerfile was never used
(pip installs pre-compiled wheels that bundle their own libraries).
Bump to Pillow ~=12.1 which ships native AVIF encoding/decoding in its
PyPI wheels out of the box.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
