[Feature] Freeze / Read-only server mode for backups

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request. • Yes The feature Quoting from the [backup guide]( > A backup of Immich should contain both the database and the asset files. When backing these up it's possible for them to get out of sync, potentially resulting in broken assets after you restore. The best way of dealing with this is to stop the immich-server container while you take a backup. If nothing is changing then the backup will always be in sync. As recommended I usually stop all the containers before backup and then start them again after backup. But if the backup is over a network (for offsite backup) or involves a whole system backup (many files, multiple services involved) it can take a long time for the backup to completely. This is especially true because Immich's backups are bursty -- most days it's only a few photos but on trips there's a LOT of photos uploaded to imm…