Implement local backup solution

[SeboCode]

The goal is to implement a backup solution independent of the service stack to quickly restore from a bakup in case of a total outage of the Pi drive / both server drives.

Requirements:

• Simplicity: The backup solution should use as little moving parts as possible in order for it to generate as little work as possible.
• Locality: The drive needs to be local. A remote solution is implemented / described in another issue.
• Security: The solution should give strong security guarantees. Access to the drive should be as minimal as possible and data should be pulled, not pushed, to improve hardening.
• Automation: The solution has to be fully automated to reduce manual toil.

Solution Proposal:

• A second external drive on the Raspberry Pi.
• Only a dedicated backup user can access the drive.
• The data transfer is initiated by the backup user to have a true pull solution.
• The job should be implemented with systemd, not as a cron job, as systemd runs after a temporary outage of the Pi.
• The backup script should initiate a wol call to the server and power off the server once done. This power off should be preventable by activating a flag in Homeassistant. A solution for this is to simply create/delete a file when clicking a button in HA and checking for that files existance before initiating the poweroff.
• The filesystem of the external drive should be ZFS, such that only incremental data from the server needs to be sent over.

Backup Strategy:

• Do everything by hand, sending raw zfs snapshot data to the Pi. This could lead to faulty sends, requiring a full resend but it is the simplest solution.
• Use syncoid to perform automated snapshot creation and syncing. It is a convenience feature but maybe overkill.
• Use sanoid for automatic snapshot management. This is most likely overkill for the start.