The SpoolBuddy remote-update flow shelled out to `ssh-keygen` to create
its update keypair on first use. Inside the Docker container the process
runs under an arbitrary PUID that is not listed in /etc/passwd, so
ssh-keygen aborted at the getpwuid() home-directory lookup with
"no user exists for uid 1001" and the update button failed.
Generate the ed25519 keypair in-process via the `cryptography` library
(already a dependency) and serialize it in OpenSSH format. No subprocess,
no /etc/passwd lookup. Native installs are unaffected.
Added a regression test that asserts no subprocess is spawned during
keypair creation so this can't come back.