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/admin/router.py

@@ -772,8 +772,20 @@ async def health_page(
             {"at": row.at, "kind": f"SMS/{row.kind}", "to": row.to_msisdn, "error": row.error}
         )
 
-    # voice pipeline heartbeat (written by the M4 pipeline service)
-    voice_status, voice_badge = "nije pokrenut (M4)", "pending"
+    # voice pipeline heartbeat (written by the voice service, M6)
+    import os
+    import time as time_mod
+
+    from gogo.voice.server import heartbeat_path
+
+    try:
+        age = time_mod.time() - os.path.getmtime(heartbeat_path())
+        if age <= 60:
+            voice_status, voice_badge = f"radi (heartbeat prije {int(age)}s)", "confirmed"
+        else:
+            voice_status, voice_badge = f"NE RADI (heartbeat star {int(age)}s)", "rejected"
+    except OSError:
+        voice_status, voice_badge = "nije pokrenut", "pending"
 
     return templates.TemplateResponse(
         request,