M6 + M0: hardening, deploy docs, PoC scripts

- Pipeline-down fallback: dialplan plays gogo-fallback after a failed
  AudioSocket (§15) + script to synthesize the Bosnian prompt via TTS
- Voice heartbeat file → /health/voice endpoint + live status in admin panel
- Retention job also purges stale call registrations; chat sessions metered;
  100%-usage super-admin alert email (GOGO_ADMIN_ALERT_EMAIL)
- .env.example, docs/DEPLOY.md (compose stack, GPU node, backups pg_dump,
  update procedure, security notes)
- §16 Definition of Done as an automated test: softphone-style call → fake
  partner availability → request pushed → webhook confirm → console SMS
- M0 PoC scripts: STT accuracy harness (CER/WER + spoken-digit check; dry-run
  verified with espeak-ng samples + faster-whisper small on CPU — phone number
  extracted exactly), TTS bake-off (Azure vs ElevenLabs, latency+cost), and
  end-to-end latency smoke test speaking real AudioSocket to the live pipeline

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gogo/api/health.py

@@ -21,3 +21,21 @@ async def health():
 async def health_db(session: AsyncSession = Depends(get_session)):
     await session.execute(text("SELECT 1"))
     return {"status": "ok"}
+
+
+@router.get("/health/voice")
+async def health_voice():
+    """Voice pipeline liveness via its heartbeat file (shared volume)."""
+    import os
+    import time
+
+    from gogo.voice.server import heartbeat_path
+
+    path = heartbeat_path()
+    try:
+        age = time.time() - os.path.getmtime(path)
+    except OSError:
+        return {"status": "down", "detail": "no heartbeat file"}
+    if age > 60:
+        return {"status": "down", "detail": f"heartbeat {int(age)}s old"}
+    return {"status": "ok", "heartbeat_age_s": int(age)}