The signing key listed for the apt repositories in the README seems to be expired.
> apt update
Get:1 https://dl.cloudsmith.io/public/caddy/stable/deb/debian any-version InRelease [14.8 kB]
Hit:2 http://security.debian.org trixie-security InRelease
Err:1 https://dl.cloudsmith.io/public/caddy/stable/deb/debian any-version InRelease
Sub-process /usr/bin/sqv returned an error code (1), error message is: Signing key on 65760C51EDEA2017CEA2CA15155B6D79CA56EA34 is bad: The subkey is not live because: Expired on 2025-12-28T21:16:00Z
Hit:3 https://download.docker.com/linux/debian trixie InRelease
Hit:4 http://deb.debian.org/debian trixie InRelease
Hit:5 http://deb.debian.org/debian trixie-updates InRelease
Hit:6 http://deb.debian.org/debian trixie-backports InRelease
Get:7 https://dl.cloudsmith.io/public/caddy/xcaddy/deb/debian any-version InRelease [14.1 kB]
Fetched 28.9 kB in 0s (137 kB/s)
All packages are up to date.
Warning: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. OpenPGP signature verification failed: https://dl.cloudsmith.io/public/caddy/stable/deb/debian any-version InRelease: Sub-process /usr/bin/sqv returned an error code (1), error message is: Signing key on 65760C51EDEA2017CEA2CA15155B6D79CA56EA34 is bad: The subkey is not live because: Expired on 2025-12-28T21:16:00Z
Warning: Failed to fetch https://dl.cloudsmith.io/public/caddy/stable/deb/debian/dists/any-version/InRelease Sub-process /usr/bin/sqv returned an error code (1), error message is: Signing key on 65760C51EDEA2017CEA2CA15155B6D79CA56EA34 is bad: The subkey is not live because: Expired on 2025-12-28T21:16:00Z
Warning: Some index files failed to download. They have been ignored, or old ones used instead.
The signing key listed for the apt repositories in the README seems to be expired.