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/jobs.py

@@ -97,6 +97,16 @@ async def run_retention_job() -> None:
                     log.exception("failed to delete recording %s", path)
                     continue
             call.recording_path = None
+
+        # drop stale dialplan handoff rows (they are only needed for minutes)
+        from sqlalchemy import delete
+
+        from gogo.models import CallRegistration
+
+        reg_cutoff = datetime.now(UTC) - timedelta(days=7)
+        await session.execute(
+            delete(CallRegistration).where(CallRegistration.created_at < reg_cutoff)
+        )
         await session.commit()
         if rows:
             log.info("retention: cleared %d recordings", len(rows))