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/server.js

@@ -26,7 +26,7 @@ const start = async () => {
   // Auto-offline mitras with stale heartbeat (every 30s)
   setInterval(async () => {
     try {
-      const count = await autoOfflineStaleMitras(45)
+      const count = await autoOfflineStaleMitras()
       if (count > 0) console.log(`Auto-offlined ${count} stale mitra(s)`)
     } catch (err) {
       console.error('Auto-offline check failed:', err)