[Server support] - Job to run a database backup

[matuella]

Create a scheduled function to run (and store) a backup of all Firestore database, allowing rollbacks if critical issues arise.

This can be done by using the Firebase Functions capabilities, alongside the Admin SDK permissions, storing it properly in a private Firebase Storage folder.

Although, not quite sure how long the timeframe of this cron job should be.

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Sub-issue that relates to the Server support - https://github.com/olmps/memo/issues/136.