v0.43.0

Busy-guard on the handset announce endpoint: an announcement is no longer
originated to a phone that is already on a call.

• POST /api/announce/{ext} now reads the target's device state
  (DEVICE_STATE(PJSIP/<ext>) via a channel-less AMI Getvar) immediately
  before the originate. When the phone is on or being offered a call
  (INUSE / RINGING / RINGINUSE / BUSY / ONHOLD), the call is not
  placed: an INVITE to a busy phone cannot auto-answer (the intercom
  Call-Info header acts only on an idle device) and instead rings the
  handset as call waiting — a phantom ring right after an alarm announcement
  when a duplicate dispatch reaches a handset still in its announce call.
• A skipped announcement is logged and reported as
  {"ok": true, "skipped": "busy", "device_state": …} (HTTP 200), so callers
  treat it as handled rather than re-queueing identical content behind the
  call in progress.
• The guard fails open — an unreadable device state (AMI hiccup) never blocks
  an announcement — and applies only to the announce endpoint: ordinary
  inbound/outbound calling, paging, test rings and wake-up calls are
  unchanged.
• New ami.get_device_state / ami.device_busy helpers, covered by
  wire-level and truth-table tests in tests/test_webui.py.