Handle Timeshift naming scheme to send its snapshots to targets

Timeshift is well integrated in my distribution to automatically create snapshot when updating the system and add them to Grub to easily "go back in time".
I would like to leverage this and only send its snapshots to backup targets with btrbk. I already use btrbk for my personal data and my docker persistant volumes and that part works well, snapshotting and backing up subvolumes.

Timeshift uses a different folder structure. It stores snapshots of `@` and `@home` as follows, in the root of the btrfs filesystem:
```
timeshift-btrfs/snapshots/2022-06-21_11-15-26/@
timeshift-btrfs/snapshots/2022-06-21_11-15-26/@home
timeshift-btrfs/snapshots/2022-06-21_11-17-54/@
timeshift-btrfs/snapshots/2022-06-21_11-17-54/@home
```
Instead of adding the timestamp to the snapshot subvolume name, it uses a timestamp in a folder that contains the snapshot :(

Is there a way I can tell btrbk.conf to look into those folders?

Start by reviewing btrbk.conf's source-subvolume path handling and how it discovers snapshots, using the Timeshift paths in the issue as the test case. Confirm whether @ and @home under timestamped directories can be selected and sent to backup targets, then document the supported configuration or implementation scope.

Written by the indexing model from the issue text.