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/app.internal.js

@@ -5,6 +5,7 @@ import { ccUserRoutes } from './routes/internal/cc-user.routes.js'
 import { rolesRoutes } from './routes/internal/roles.routes.js'
 import { internalAuthRoutes } from './routes/internal/auth.routes.js'
 import { internalConfigRoutes } from './routes/internal/config.routes.js'
+import { sessionManagementRoutes } from './routes/internal/session.routes.js'
 import { errorHandler } from './plugins/error-handler.js'
 
 export const buildInternalApp = async () => {
@@ -18,6 +19,7 @@ export const buildInternalApp = async () => {
   app.register(ccUserRoutes, { prefix: '/internal/control-center-users' })
   app.register(rolesRoutes, { prefix: '/internal/roles' })
   app.register(internalConfigRoutes, { prefix: '/internal/config' })
+  app.register(sessionManagementRoutes, { prefix: '/internal/sessions' })
 
   return app
 }