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/services/dashboard.service.js

@@ -0,0 +1,28 @@
+import { getDb } from '../db/client.js'
+
+const sql = getDb()
+
+export const getDashboardStats = async () => {
+  const [[{ active_chats }], [{ online_mitras }], [{ pending_requests }]] = await Promise.all([
+    sql`SELECT COUNT(*) AS active_chats FROM chat_sessions WHERE status IN ('active', 'pending_payment')`,
+    sql`SELECT COUNT(*) AS online_mitras FROM mitra_online_status WHERE is_online = true`,
+    sql`SELECT COUNT(*) AS pending_requests FROM chat_sessions WHERE status IN ('searching', 'pending_acceptance')`,
+  ])
+
+  const customersPerMitra = await sql`
+    SELECT m.id, m.display_name,
+      (SELECT COUNT(*) FROM chat_sessions cs
+       WHERE cs.mitra_id = m.id AND cs.status IN ('active', 'pending_payment')) AS active_session_count
+    FROM mitras m
+    INNER JOIN mitra_online_status s ON s.mitra_id = m.id
+    WHERE s.is_online = true
+    ORDER BY active_session_count DESC
+  `
+
+  return {
+    active_chats: Number(active_chats),
+    online_mitras: Number(online_mitras),
+    pending_requests: Number(pending_requests),
+    customers_per_mitra: customersPerMitra,
+  }
+}