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/services/mitra-status.service.js

@@ -1,5 +1,6 @@
 import { getDb } from '../db/client.js'
 import { SessionStatus } from '../constants.js'
+import { getMitraPingConfig } from './config.service.js'
 
 const sql = getDb()
 
@@ -58,7 +59,12 @@ export const getStatus = async (mitraId) => {
     FROM mitra_online_status
     WHERE mitra_id = ${mitraId}
   `
-  return status
+  const pingConfig = await getMitraPingConfig()
+  return {
+    ...status,
+    require_ping: pingConfig.require_ping,
+    ping_interval_seconds: pingConfig.ping_interval_seconds,
+  }
 }
 
 export const getOnlineMitras = async () => {
@@ -89,7 +95,13 @@ export const getOnlineLogs = async (mitraId, { page = 1, limit = 50 } = {}) => {
   return { items, total: Number(count), page, limit }
 }
 
-export const autoOfflineStaleMitras = async (staleSeconds = 45) => {
+export const autoOfflineStaleMitras = async () => {
+  const pingConfig = await getMitraPingConfig()
+
+  // If ping is not required, skip the auto-offline sweep entirely
+  if (!pingConfig.require_ping) return 0
+
+  const staleSeconds = pingConfig.ping_interval_seconds * 3
   const stale = await sql`
     UPDATE mitra_online_status
     SET is_online = false, last_offline_at = NOW(), updated_at = NOW()