Write a systemd service for the server

Pricewarp runs from an executable currently but there's no easy way to start the service, stop it, enable it, disable it, update it, etc. We should write a systemd service for it and make it easy to copy over patches for the executable so it can be updated seamlessly.

Start by locating how the Pricewarp executable is built and launched, then review the repository's existing deployment or operational files, if any. Define the systemd service around the executable and document or test the intended start, stop, enable, disable, and seamless update workflow; done means those operations are practical and the executable can receive patches without avoidable interruption.

Written by the indexing model from the issue text.