Replaces the per-frame asyncio.Event signaling with a monotonic
timestamp updated on each audio frame. The handler sleeps until the
next deadline (last_audio_time + timeout), recomputing on each wake-up
to account for audio arriving during sleep.
This avoids waking the handler on every audio frame (~50/s at 20ms
chunks), and guarantees detection latency is bounded by timeout rather
than 2 * timeout.
Also renames audio_starvation_timeout to audio_idle_timeout and
associated identifiers for consistency with existing pipecat naming
(user_idle_timeout, etc.).
