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/session-timer.service.js

@@ -1,6 +1,7 @@
 import { getDb } from '../db/client.js'
 import { publish } from '../plugins/valkey.js'
 import { sendToSessionParticipant } from '../plugins/websocket.js'
+import { sendPushNotification } from './notification.service.js'
 import { UserType, SessionStatus, WsMessage } from '../constants.js'
 
 const sql = getDb()
@@ -85,15 +86,29 @@ const onSessionExpired = async (sessionId) => {
   `
   if (!session) return
 
-  // Notify customer — sees extend/close dialog
+  // Notify customer — sees extend/close dialog; FCM fallback if WebSocket is down
   const expiredData = { type: WsMessage.SESSION_EXPIRED, session_id: sessionId }
-  sendToSessionParticipant(sessionId, UserType.CUSTOMER, expiredData)
+  const customerSent = sendToSessionParticipant(sessionId, UserType.CUSTOMER, expiredData)
+  if (!customerSent) {
+    await sendPushNotification(UserType.CUSTOMER, session.customer_id, {
+      title: 'Waktu Sesi Habis',
+      body: 'Sesi curhat kamu telah habis. Ketuk untuk memperpanjang atau mengakhiri.',
+      data: { type: WsMessage.SESSION_CLOSING, session_id: sessionId },
+    })
+  }
 
   // Notify mitra — sees expired + closing (waits for customer's decision or goodbye)
-  sendToSessionParticipant(sessionId, UserType.MITRA, expiredData)
+  const mitraSent = sendToSessionParticipant(sessionId, UserType.MITRA, expiredData)
   sendToSessionParticipant(sessionId, UserType.MITRA, {
     type: WsMessage.SESSION_CLOSING, session_id: sessionId,
   })
+  if (!mitraSent) {
+    await sendPushNotification(UserType.MITRA, session.mitra_id, {
+      title: 'Sesi Berakhir',
+      body: 'Sesi curhat telah berakhir. Ketuk untuk menulis pesan penutup.',
+      data: { type: WsMessage.SESSION_CLOSING, session_id: sessionId },
+    })
+  }
 
   // Also publish via Valkey for any listeners
   await publish(`session:${sessionId}:status`, expiredData)