Phase 2 scaffold: mitra online status & pairing logic

Add mitra online/offline status with heartbeat-based auto-offline,
customer-mitra pairing via Valkey pub/sub blast, session management,
and control center dashboard with real-time stats.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/package.json

@@ -14,6 +14,7 @@
     "fastify": "^4.28.1",
     "@fastify/sensible": "^5.6.0",
     "firebase-admin": "^12.2.0",
+    "ioredis": "^5.4.1",
     "pg": "^8.12.0",
     "postgres": "^3.4.4",
     "zod": "^3.23.8",