Add Systemd config for self-hosted server

[svenstaro]

Fixes #1864.

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• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[ellie]

The env makes sense, but it might be better to just use the config file. We autocreate with the defaults which hopefully leads to less people asking for help with things

Setting ATUIN_CONFIG_DIR=/etc/atuin and ensuring that directory is created would make that work