Make daemon stop behavior configurable via signal_ups_on_stop

Off (default): SIGKILL the daemon on addon stop so the UPS keeps power
on. On: SIGTERM it (native behavior) so it saves the RTC and tells the
UPS to cut power. Add translations so the option is documented in the
HA config UI, and document the difference in the README.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -32,12 +32,19 @@ LIFEPO4WERED_FAKE=1 MQTT_HOST=... python3 hassio-addon-lifepo4wered/lifepo4wered
 
 ## Stopping the addon vs. shutting down
 
-Stopping the addon does **not** power off the Pi: the daemon is killed
-without notifying the UPS, so power stays on (`run.sh` traps the stop
-signal — the daemon would otherwise report "system shutting down" and the
-UPS would cut power seconds later). While the addon is stopped, UPS-side
-features (button shutdown, low-battery clean shutdown, boot watchdog
-handshake) are inactive until it starts again.
+The `signal_ups_on_stop` option controls what happens to the daemon when
+the addon is stopped:
+
+- **Off (default)** — the daemon is killed (SIGKILL) without notifying the
+  UPS, so power stays on. Addon stops, restarts and updates are safe. The
+  system clock is not saved to the UPS RTC on stop (it still is on
+  UPS-commanded shutdowns), and while the addon is stopped, UPS-side
+  features (button shutdown, low-battery clean shutdown) are inactive.
+- **On** — the daemon is terminated gracefully (SIGTERM), the native
+  LiFePO4wered behavior: it saves the clock to the UPS RTC and reports
+  "system shutting down" (`PI_RUNNING = 0`), so the UPS **cuts power to
+  the Pi a few seconds after the addon stops**. Only use this if you stop
+  the addon exclusively as part of powering the system down.
 
 When the UPS itself commands a shutdown (button press, low battery,
 `AUTO_SHDN_TIME` after power loss), the addon performs a clean host