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/db/migrate.js

@@ -274,6 +274,20 @@ const migrate = async () => {
     ON CONFLICT (key) DO NOTHING
   `
 
+  // --- Phase 3.1: Mitra Ping Config ---
+
+  await sql`
+    INSERT INTO app_config (key, value)
+    VALUES ('require_mitra_ping', '{"value": true}')
+    ON CONFLICT (key) DO NOTHING
+  `
+
+  await sql`
+    INSERT INTO app_config (key, value)
+    VALUES ('mitra_ping_interval_seconds', '{"value": 15}')
+    ON CONFLICT (key) DO NOTHING
+  `
+
   console.log('Migration complete.')
   await sql.end()
 }