Change storage box size from 10000 to 10240 GiB.

Increase default time range from 90 days to 6 months.

Mark "Free Space (Hetzner)" metric as instant for faster updates.

As it turns out, the total size does not include the snapshots (feature
which we recently enabled). Record the free space so we can have a more
accurate picture of the usable space remaining.

Now that we have enabled snapshots, summing all Borg repository sizes
has become a bad approximation of the total space used on the storage
box.

Currently only tracks repo sizes and total backup space used (as a sum
of the former, not the real exact space used on each storage location).