feat(bridge): gate Whisper behind Silero VAD; harden broadcast auto-start

Address review of the noise/broadcast fixes:

- STT now refuses to run Whisper on non-speech. transcribe() runs the Silero
  VAD (bundled with faster-whisper, no new dep) BEFORE the model, so noise or a
  brief loud blip with no real speech never reaches STT and can't be
  hallucinated into a transcript. The no_speech_prob/avg_logprob post-filter
  stays as a second line of defence (a clap the VAD lets through is still killed
  by Whisper's own no_speech_prob). VAD is env-tunable (VAD_THRESHOLD,
  VAD_MIN_SPEECH_MS, VAD_ENABLED) and fail-open so a VAD error never swallows a
  real utterance. Validated on real audio: synthesised Korean speech passes;
  silence, a 50ms blip and white noise are rejected.

- Broadcast auto-start no longer blocks the voice join and no longer silently
  swallows failures: wiring is synchronous, the Go-Live start runs in the
  background with a bounded retry and a loud final-failure log.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_bridge_stt_filter.py

@@ -7,15 +7,39 @@ produces no transcript and no reply. Thresholds are config-driven, so the tests
 pass explicit references rather than hardcoding the production defaults.
 """
 
+import numpy as np
 import pytest
 
 from bridge.stt_filter import (
     filter_speech_segments,
+    has_speech,
     is_non_speech,
     segment_confidence,
 )
 
 
+@pytest.mark.unit
+def test_has_speech_rejects_silence():
+    # Pure digital silence is unambiguously non-speech: VAD must skip STT.
+    silence = np.zeros(16000, dtype=np.float32)  # 1s @ 16kHz
+    assert has_speech(silence, 16000) is False
+
+
+@pytest.mark.unit
+def test_has_speech_rejects_a_brief_loud_blip():
+    # A short loud transient (a clap / pop): mostly silence with a 50ms spike,
+    # below the min-speech duration, so no real speech region is found.
+    audio = np.zeros(16000, dtype=np.float32)
+    audio[8000:8800] = 0.9  # ~50ms full-scale burst
+    assert has_speech(audio, 16000, min_speech_duration_ms=200) is False
+
+
+@pytest.mark.unit
+def test_has_speech_fails_open_on_empty_when_vad_present_returns_false():
+    # Empty audio has nothing to transcribe; treat as non-speech.
+    assert has_speech(np.zeros(0, dtype=np.float32), 16000) is False
+
+
 class Seg:
     """Minimal stand-in for a faster-whisper segment."""