Add per-server backup storage and per-backup size limits

[JainaGam3r45]

• The current backup limit only controls the number of backups.
• A small number of large backups can still fill the node or backup storage.
• This can affect other servers on the same node and create avoidable operational issues.
• Admins should be able to configure a total backup storage quota per server.
• Admins should also be able to configure a maximum size for each individual backup.
• A value of 0 should keep the current behavior and mean unlimited.
• Scheduled backups could rotate old unlocked backups when quota is reached.
• Locked backups should never be removed automatically.
• The Panel can enforce final reported backup sizes today.
• If maintainers want hard prevention while the archive is being generated, this may also require a Wings-side limit.
• Ask whether this is acceptable as a Panel-first improvement or whether they prefer Panel + Wings changes together.

[AndyIsHereBoi]

what should happen if the backup is over the limit?

also did you write this with ai? im not sure what you mean by this

If maintainers want hard prevention while the archive is being generated, this may also require a Wings-side limit.
Ask whether this is acceptable as a Panel-first improvement or whether they prefer Panel + Wings changes together.