v0.5.0

• device battery: network-wide battery audit. One <base>/# subscription
  collects every retained device state and joins battery (percent),
  battery_low and voltage (mV) onto the bridge/devices inventory — no
  per-device round trip. Rows sort worst-first (low → unknown → ok,
  then percent ascending) so the top row is the device that needs a fresh
  cell; mains-powered devices and the coordinator are dropped, and battery
  devices that never published state (sleeping sensors) classify as
  unknown. --below N redefines "low" (default 20%), --low-only drops
  healthy devices, --timeout tunes the inventory read, --duration the
  state-collection window. Core functions battery_sweep and
  classify_battery in core/devices.py. 37 new tests (unit + E2E +
  workflow); total coverage 99.8%.