feat(systemd): monthly btrfs balance template

Template service+timer that runs `btrfs balance start -dusage=50
-musage=50 %f` once a month on the instance's mount path. Mirrors the
shape of the stock btrfs-scrub@.{service,timer} so the operational
model is identical: enable btrfs-balance@-.timer for /, btrfs-balance@\
x2dhome.timer for /home, etc.

Why a partial balance and not a full one: full `btrfs balance start`
rewrites every block group, which on a multi-TB volume takes hours and
can chew through enormous amounts of CSUM/free-space-tree work.
`-dusage=50 -musage=50` only consolidates block groups that are less
than half full, which is exactly the operation that reclaims space
'lost' to fragmentation after lots of small writes — the only practical
reason a healthy single-disk btrfs needs balancing at all.

`Nice=19 IOSchedulingClass=idle` keeps it out of the way of foreground
work; `KillSignal=SIGINT` (same as btrfs-scrub) lets a graceful Ctrl-C
checkpoint the operation cleanly. Persistent=true catches the run on
next boot if the machine was off when the timer fired.

Enabled in systemd-units/system.txt as btrfs-balance@-.timer (root
volume only — /home isn't a separate subvolume on this machine).

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