POST /api/live/calls/:linkedid/spy {mode} per 04 §7.3: rings the
admin's desk phone (409 if it isn't connected, 400 without an
extension) and, when they answer, runs ChanSpy on the call's first
user leg (never a trunk or Local channel, never the admin's own),
which for an inbound or queue call is the agent. Options qE / qEw /
qEB: no beep or name, the spy leg ends with the call, whisper to that
user only, or barge (both sides hear). Audited as call_spy.
Dashboard: Listen / Whisper / Barge on talking calls with a user on
them, and "your desk phone is ringing" once sent. The spy leg shows as
its own call; its Hang up stops listening.
e2e m10-t4: 380 calls 381, the admin's 382 runs ChanSpy with each
mode's options, and the barge leg ends with the call. The roadmap
records that M10's milestone check (real Yealink/Grandstream, Linphone
scanning the QR) waits for hardware the owner doesn't have yet.
Task: M10-T4