The systemd unit pins PATH to the venv only, so subprocess.run(['pandoc', ...])
raised FileNotFoundError even though pandoc was installed at /usr/bin/pandoc.
The handler's "pandoc not installed" message was misleading — pandoc was
reachable from a login shell but not from the service. Rephrased to point at
the actual cause: the service's PATH. The systemd drop-in to extend PATH is
not committed here (lives at /etc/systemd/system/aaronai.service.d/path.conf
on the host).