[HOWTO] Setup archive targets in btrbk.conf

# HOWTO: set up complex, persistent archive configuration in `btrbk.conf`

After some deep man page reading and lateral thinking, here's how I setup archiving to avoid needing to specify all the config at the command line.

Features:

- Prevents snapshot creation (`snapshot_create no`)
- Not run by default (`noauto yes`)
- Preserves all source snapshots
- Allows different retention schedules for different subvolumes
- Labelled `group` statements, so be run via `btrbk run nas`, or sub-sections only: `btrbk run nas-home

## Overview
This method makes uses two configuration sections:

### The usual snapshot/backup config section (nothing new here, just check the terms I use):
- Source: live filesystem
- Snapshot target: another directory on the live filesystem
- Backup target: another filesystem. Below, I'll call this the *backup disk*

### The archive config section
- Source: backup disk
- Snapshot target: DISABLED via `snapshot_create no`, but needs to be specified as a source for the backup target
- Backup target: the archive repository, eg a NAS or S3 storage mounted as a filesystem

## Example

I'm a digital nomad. My usual backup target is an external HDD. I also want to save backups on a NAS that resides at my parent's house.

I have 2 filesystems to backup:
- rootfs (subvolume `@`)
- home, with two subvols:
- `@home` -- the bulk of it
- `@home-var` -- `.cache` folders and data that could be regenerated, eg downloads, blockchains. I symlink these type of folders into this subvolume's mountpoint.

All my subvols get backed up to my external HDD `/media/ssd/btrbk`:

```
volume /mnt/btrfs-vol/rootfs
group svelte
subvolume @
group rootfs
snapshot_dir btrbk/ # Seems to default to /btrbk
# snapshot_name defaults to , but be explicit:
snapshot_name rootfs

target send-receive /media/ssd/btrbk/rootfs

volume /mnt/btrfs-vol/home
group svelte

subvolume @home
group home
snapshot_dir btrbk/home
snapshot_name home

target send-receive /media/ssd/btrbk/home

subvolume @home@var
group home-var
snapshot_dir btrbk/home-var
snapshot_name home-var
snapshot_preserve_min 36h
# Ensure that these are preserved long enough to be picked up by sporadic runs of the `nas` groups below.
snapshot_preserve 48h 14d 6w 3m

target send-receive /media/ssd/btrbk/home-var
```

And now from external HDD `/media/ssd/btrbk` to NAS:

```
# Using a hack here - we never want to 'run' these profiles to create new snapshots.
# The snaps are created above. This section is for copying from the above section's backup target to archive target.

volume /media/ssd/btrbk
group nas # Add this whole section's scope for command line filtering
ssh_user root # FIXME: change to btrbk

# Don't create snapshots
noauto yes # Only run this section if explicitly included in a filter
snapshot_create no # Skip snapshot creation

snapshot_preserve *h # Don't delete backup disk snaps at all
snapshot_preserve_min all # Don't delete backup disk snaps at all

target_preserve 52w *m # Archive retention policy
target_preserve_min no # Don't create a non-preserved backups

subvolume home
group nas-home # for command line filtering
snapshot_dir home
snapshot_name home

target send-receive ssh://nas:5090/btrbk/svelte/home

subvolume rootfs
group nas-rootfs # for command line filtering
snapshot_dir rootfs
snapshot_name root

target send-receive ssh://nas:5090/btrbk/svelte/rootfs

subvolume home-var
group nas-home-var # for command line filtering
snapshot_dir home-var

target send-receive ssh://nas:5090/btrbk/svelte/home-var
```