v0.8.0

• device ping: the bridge liveness round trip. Publishes {"id": ...} to
  bridge/request/device/ping; z2m performs a real Zigbee read on the
  device's basic cluster and reports data.successful. This is the third —
  and only active — liveness view: device availability reads a retained
  flag the device last published, device stale reads raw last_seen, but
  only device ping proves the radio link works right now.
  • IDENT accepts a friendly name or IEEE address; unknown devices abort
    before any publish (same as device identify).
  • --timeout S (default 15) tunes how long the bridge gets to answer.
  • A bridge timeout or z2m error surfaces as successful: false with an
    error field — never a traceback — and --json output stays parseable,
    so scripts can gate on it: the command exits 1 when the device does
    not answer.
  • The flip side, stated in the help text: a sleeping battery device will
    not answer a ping even when perfectly healthy — use
    device availability there. device ping + device availability /
    availability-sweep now cover both halves of "is it alive?".
• Core function ping in core/devices.py.
• Why it matters: after a device availability-sweep --offline-only or a
  firmware upgrade, device ping <name> is the one-liner that separates
  "flag says offline" from "the device really does not answer".
• 15 new tests (6 unit + 9 E2E/workflow); total suite 991 passed, gate
  green (coverage 99.3%, ruff check/format clean, bandit clean).