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/telephony/asterisk.py

@@ -156,6 +156,9 @@ exten => _[+0-9].,1,NoOp(Gogo inbound ${{EXTEN}} for {slug})
  same => n,Dial(${{TARGETS}},${{RINGTIME}})
  same => n,GotoIf($["${{DIALSTATUS}}"="ANSWER"]?answered)
  same => n(agent),AudioSocket(${{GOGO_UUID}},${{GOGO_VOICE}})
+ ; if the voice pipeline is down AudioSocket fails instantly → fallback (§15);
+ ; after a normal agent call the caller is gone and Playback is a no-op
+ same => n,Playback(gogo-fallback)
  same => n,Hangup()
  same => n(answered),Set(RES=${{CURL(${{GOGO_BACKEND}}/internal/calls/${{GOGO_UUID}}/answered?token=${{GOGO_TOKEN}}&duration=${{CDR(billsec)}})}})
  same => n,Hangup()