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/config.service.js

@@ -15,3 +15,17 @@ export const setAnonymityConfig = async (enabled) => {
   `
   return { anonymity_enabled: enabled }
 }
+
+export const getMaxCustomersPerMitra = async () => {
+  const [row] = await sql`SELECT value FROM app_config WHERE key = 'max_customers_per_mitra'`
+  return { max_customers_per_mitra: row?.value?.value ?? 3 }
+}
+
+export const setMaxCustomersPerMitra = async (value) => {
+  await sql`
+    INSERT INTO app_config (key, value, updated_at)
+    VALUES ('max_customers_per_mitra', ${sql.json({ value })}, NOW())
+    ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value, updated_at = NOW()
+  `
+  return { max_customers_per_mitra: value }
+}