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/src/plugins/valkey.js

@@ -0,0 +1,47 @@
+import Redis from 'ioredis'
+
+let pub
+let sub
+let client
+
+export const getValkeyClient = () => {
+  if (!client) {
+    const url = process.env.VALKEY_URL || 'redis://localhost:6379'
+    client = new Redis(url)
+  }
+  return client
+}
+
+export const getValkeyPub = () => {
+  if (!pub) {
+    const url = process.env.VALKEY_URL || 'redis://localhost:6379'
+    pub = new Redis(url)
+  }
+  return pub
+}
+
+export const getValkeySub = () => {
+  if (!sub) {
+    const url = process.env.VALKEY_URL || 'redis://localhost:6379'
+    sub = new Redis(url)
+  }
+  return sub
+}
+
+export const publish = async (channel, data) => {
+  const pubClient = getValkeyPub()
+  await pubClient.publish(channel, JSON.stringify(data))
+}
+
+export const subscribe = (channel, callback) => {
+  const subClient = getValkeySub()
+  subClient.subscribe(channel)
+  subClient.on('message', (ch, message) => {
+    if (ch === channel) {
+      callback(JSON.parse(message))
+    }
+  })
+  return () => {
+    subClient.unsubscribe(channel)
+  }
+}