battery: alert on mains<->battery transitions

Pushover notification when the router loses mains (now on battery) and when
mains is restored. One alert per real transition; first sample is skipped so a
restart while already on battery doesn't false-fire. Independent of the
low-charge 25/20/15/10% alerts.

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

power/van-battery

@@ -177,15 +177,26 @@ def main():
         on_batt = read_on_battery(cfg)
         cap = read_capacity(cfg)
 
+        capstr = f"{cap}%" if cap is not None else "unknown charge"
+
+        # Mains <-> battery transition alerts. Skip the very first sample (prev is None)
+        # so a restart while already on battery doesn't fire a spurious "on battery".
+        if on_batt is not None and prev_on_batt is not None and on_batt != prev_on_batt:
+            if on_batt:
+                log(f"mains lost — running on battery at {capstr}", "warn")
+                pushover(cfg, f"⚡ {HOST}: on battery",
+                         f"Mains power lost — now running on battery ({capstr}). "
+                         f"Low alerts at {cfg['warn_levels']}%, auto-shutdown at {shutdown_level}%.")
+            else:
+                log(f"mains restored at {capstr} — alerts re-armed")
+                pushover(cfg, f"🔌 {HOST}: back on mains",
+                         f"Mains power restored ({capstr}). Battery alert sequence re-armed.")
+
         if on_batt is False:
             # On mains: re-arm the whole sequence for the next discharge episode.
-            if prev_on_batt:
-                log("mains restored — alerts re-armed")
             last_alerted = None
             shutdown_issued = False
         elif on_batt is True and cap is not None:
-            if prev_on_batt is False:
-                log(f"mains lost — running on battery at {cap}%", "warn")
             sev = severity(cap, levels)
             if sev is not None and (last_alerted is None or sev < last_alerted):
                 last_alerted = sev