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/pairing.service.js

@@ -19,8 +19,8 @@ const notifyMitra = async (mitraId, data) => {
     if (data.type === WsMessage.CHAT_REQUEST) {
       await sendPushNotification(UserType.MITRA, mitraId, {
         title: 'Permintaan Chat Baru',
-        body: 'Ada pelanggan yang ingin curhat!',
-        data: { type: WsMessage.CHAT_REQUEST, session_id: data.session_id },
+        body: 'Ada pelanggan yang ingin curhat! Ketuk untuk menerima.',
+        data: { type: WsMessage.CHAT_REQUEST, session_id: data.session_id, action: 'open_accept' },
       })
     }
   }