btrbk: local snapshots of @home

system/services/btrbk.nix. The minutes-scale "undo".

It is NOT a backup

A snapshot lives on the SAME disk as the data. The Kingston dies, the snapshots die with it. What covers that is restic (docs/notes/restic.md), off-disk on the Seagate plus the Drive. Both exist because they answer different questions:

	answers	cadence
restic	"the disk died / the house burned down"	daily, off-disk, encrypted
btrbk	"I overwrote the file 20 minutes ago"	hourly, instant, local

restic alone leaves a hole of up to 24 h and a restore that takes minutes. btrbk closes that hole for ~zero cost, because a CoW snapshot copies nothing: it only starts taking space to the EXTENT that the original data diverges.

@home only

The root is left out on purpose. On NixOS the system's rollback already IS the GRUB generation list, and a snapshot of / would not even take /nix (a separate subvolume, since a snapshot does not descend into a nested one). Noise with no gain.

The prerequisite

The @snapshots subvolume mounted at /.snapshots (see docs/notes/disko.md). On an already installed system it is created by hand ONCE; the command is over there.

RequiresMountsFor = "/.snapshots" is the same lock restic uses: without it mounted, btrbk would write inside @, the one place where impermanence erases everything, and without the owner noticing.

The settings that are not obvious

• snapshot_create = "onchange": nothing written since the last snapshot means no new one. Without it the machine on and idle would generate 24 identical snapshots a day and push the useful ones out of the retention window.
• snapshot_preserve = "48h 7d 4w": about 2 days of fine granularity plus a month of safety net. It matches restic's --keep-daily 7 --keep-weekly 4, so btrbk covers exactly what is too short for the daily backup to reach. snapshot_preserve_min = "latest" guarantees it never ends up with NO snapshot at all.
• timestamp_format = "long" because an hourly snapshot needs hour:minute in the name.
• The ABSOLUTE PATH form, with no volume section. btrbk's other form (volume <pool> plus a relative subvolume) presumes subvolid=5 mounted in a directory, and mounting the top permanently would make every subvolume appear TWICE in the tree, which confuses du, find and any sweep. With an absolute path btrbk resolves /home directly.
• Persistent = true on the timer already comes from the btrbk module. It matters on this machine because it reboots a lot, but there is no need to repeat it.

Restoring

A file is a cp from /.snapshots/home.<timestamp>/..., since a snapshot is just a browsable directory. Reverting the WHOLE @home is a manual, conscious operation (swapping the subvolume), never automatic.