[admin] server db size warning threshold

[mikeldking]

we should be able to set a warning threshold such that admins would be notified that capacity is exceeded and retention policies need to be adjusted.

[mikeldking]

Note that it might make sense to automatically set this based on some sort of heuristic at first - e.g. if the Max is X, we might want to make sure there's enough capacity to run migrations. E.g. something like 90% being the default.