Add a backup site feature

It would need an API to backup a site (the command would only be accessible for site having Admin permissions), and it would do a complete copy of the site's directory and later if you want you could browse the site's backup. And if you make a new backup, it could do two things:
- override the old backup
- create another backup, in this case the backups would need to be timestamped, and it could also be incremental backups, it would decrease the size significantly

Me, I prefer the second solution

No files, tests, or entry points are named. Start by locating the site's API and Admin permission handling, then define the backup scope, browsing behavior, replacement versus timestamped storage, and incremental-backup expectations before implementation.

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