monitoring: RpiSofiaUndervoltage alerts on new brown-out, not until reboot

The rpi-sofia under-voltage alert keyed off the sticky firmware bit
(rpi_under_voltage_occurred == 1), which latches on the first brown-out and
stays 1 until the Pi reboots. With alert-on-change routing it re-paged on every
boot cycle and sat firing for ~211h of the last 14d — Viktor reported "getting a
few of these lately" — and it disagreed with the HA-sofia dashboard, which shows
the live state and reads OK once voltage recovers.

Can't just switch to the live bit: rpi_under_voltage_now never registered once in
14d (brown-outs are sub-second and fall between the 1-min textfile-collector
samples), so the sticky bit is the only reliable detector.

Fix: edge-trigger on a NEW latch via increase(rpi_under_voltage_occurred[1h]) > 0.
Fires once per brown-out and auto-resolves ~1h later (~2h active over the same
14d instead of ~211h); counter-reset handling makes a clean reboot a no-op. Both
real brown-out events in the window are still caught. Docs updated in the same
commit (monitoring.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stacks/monitoring/modules/monitoring/prometheus_chart_values.tpl

@@ -879,12 +879,17 @@ serverFiles:
             annotations:
               summary: "rpi-sofia rootfs is READ-ONLY — failing SD card (the silent-journal failure mode from this incident). Reflash/replace the card."
           - alert: RpiSofiaUndervoltage
-            expr: rpi_under_voltage_occurred{instance="rpi-sofia"} == 1
+            # Edge-trigger on the sticky firmware bit (rpi_under_voltage_now is too
+            # transient to catch at 1-min sampling — 0 hits in 14d). Fires on a NEW latch
+            # and auto-resolves ~1h later instead of latching until reboot; counter-reset
+            # handling makes a clean reboot a no-op. Since-boot state + history: Grafana "RPi Sofia".
+            expr: increase(rpi_under_voltage_occurred{instance="rpi-sofia"}[1h]) > 0
             for: 5m
             labels:
               severity: warning
             annotations:
-              summary: "rpi-sofia under-voltage detected since last boot — check PSU/USB power cable"
+              summary: "rpi-sofia under-voltage event in the last hour — check PSU/USB power cable + SD card"
+              description: "A new under-voltage brown-out latched on rpi-sofia within the last 1h (repeat/sustained events risk SD-card corruption). Sticky since-boot flag + history on Grafana 'RPi Sofia' (Hardware)."
           - alert: RpiSofiaHighTemp
             expr: rpi_soc_temp_celsius{instance="rpi-sofia"} > 75
             for: 10m