rewrite as a daemon with shutdown on low battery

the oneshot+timer produced 8367 journal lines a day of systemd
start/stop noise. now Type=notify with an internal ticker, logging
transitions only.

hardware sits behind sensor.Sensor; internal/monitor is pure, so the
shutdown state machine is tested without a pi.

four guards, each tested, because a false poweroff of a box with no
physical access is worse than a missed one:
- a failed read resets the arming window rather than pausing it
- voltage must actually fall across the window, else it is a stuck
  sensor. this is the only defence against the ac line reading 0
  while mains is connected, so it is load-bearing
- a settle period stops a restart loop acting on early readings
- flapping ac cannot accumulate

verified on hardware 2026-08-09: after poweroff on battery the x1208
starts the pi again when mains returns, with no button press. that
was the risk that could have made this feature unsafe.

a failed tick rewrites the previous sample with sensor_healthy 0 and
the failure counter, so last_update still freezes for the staleness
alert while the failure stays visible.

module path moved to git.mial.net. debian units and the compose
overlay deleted; deployment lives in the infra repo.

go.mod

@@ -1,4 +1,4 @@
-module github.com/mokhtar/x1208-exporter
+module git.mial.net/mokhtar/x1208-exporter
 
 go 1.23
 
@@ -6,3 +6,5 @@ require (
 	github.com/warthog618/go-gpiocdev v0.9.1
 	golang.org/x/sys v0.27.0
 )
+
+require github.com/coreos/go-systemd/v22 v22.7.0