Phase 3.1 WS2: Backend FCM fallback, ping config, unread API

- Add require_mitra_ping + mitra_ping_interval_seconds config keys (migration)
- Add getMitraPingConfig/setMitraPingConfig to config service
- Add GET/PATCH /internal/config/mitra-ping routes for control center
- Update mitra status service: honor ping config in auto-offline sweep,
  include ping config in GET /api/mitra/status response
- Enhance pairing FCM payload with action: 'open_accept' for deep-link
- Add FCM fallback to closure.service (initiateEarlyEnd, completeSession)
- Add FCM fallback to session-timer.service (onSessionExpired)
- Add unread count queries (getActiveSessionByCustomerWithUnread,
  getActiveSessionsByMitraWithUnread)
- Add GET /api/client/chat/session/active-with-unread route
- Add GET /api/mitra/chat-requests/sessions/active-with-unread route

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

backend/src/routes/internal/config.routes.js

@@ -6,6 +6,7 @@ import {
   getFreeTrialConfig, setFreeTrialConfig,
   getExtensionTimeoutConfig, setExtensionTimeoutConfig,
   getEarlyEndConfig, setEarlyEndConfig,
+  getMitraPingConfig, setMitraPingConfig,
 } from '../../services/config.service.js'
 
 const attachCcUser = async (request, reply) => {
@@ -102,6 +103,28 @@ export const internalConfigRoutes = async (app) => {
     return reply.send({ success: true, data: config })
   })
 
+  // --- Phase 3.1: Mitra Ping Config ---
+  app.get('/mitra-ping', {
+    preHandler: [authenticate, attachCcUser, requirePermission('config', 'read')],
+  }, async (request, reply) => {
+    const config = await getMitraPingConfig()
+    return reply.send({ success: true, data: config })
+  })
+
+  app.patch('/mitra-ping', {
+    preHandler: [authenticate, attachCcUser, requirePermission('config', 'update')],
+  }, async (request, reply) => {
+    const { require_ping, ping_interval_seconds } = request.body ?? {}
+    if (require_ping !== undefined && typeof require_ping !== 'boolean') {
+      return reply.code(422).send({ success: false, error: { code: 'VALIDATION_ERROR', message: 'require_ping must be a boolean' } })
+    }
+    if (ping_interval_seconds !== undefined && (typeof ping_interval_seconds !== 'number' || ping_interval_seconds < 5)) {
+      return reply.code(422).send({ success: false, error: { code: 'VALIDATION_ERROR', message: 'ping_interval_seconds must be a number >= 5' } })
+    }
+    const config = await setMitraPingConfig({ require_ping, ping_interval_seconds })
+    return reply.send({ success: true, data: config })
+  })
+
   // --- Price Tiers ---
   app.get('/price-tiers', {
     preHandler: [authenticate, attachCcUser, requirePermission('config', 'read')],