fix(backup): satisfy clippy::unnecessary_sort_by on snapshot ordering

Both descending sorts over safety snapshots compared an `i64` key by hand,
which `clippy::unnecessary_sort_by` rejects under `-D warnings` and broke
the CI lint. They now sort by `std::cmp::Reverse` of that key.

Ordering is unchanged: the key is `Copy`, and `sort_by_key` is stable like
`sort_by`, so equal timestamps keep their `read_dir` order.