Fix Socket.io Real-Time Issues

I have a Node.js application that relies on Socket.io v2.x for live updates, room-based messaging, and general bidirectional data sync. The core logic works when tested in isolation, yet the moment I spin up the full stack the sockets behave unpredictably—connection events fire sporadically, clients drop without explanation, and sometimes nothing is received at all even though the server shows no explicit errors.

Key facts you should know:
• The project is locked to Socket.io v2.x and must stay on that branch for legacy reasons.
• Server logs show “connected” and “disconnected” events in quick succession or not at all; there are no stack traces, only unstable connections.
• CORS is currently configured with custom rules that whitelist two specific origins.

What I need from you is a focused diagnostic and fix. Trace the handshake flow, verify the CORS headers, confirm the transport upgrade path (websocket ⇄ polling), and deliver code-level changes or configuration tweaks that restore stable, real-time messaging.

Acceptance criteria:
1. Clients connect and remain connected for a 30-minute idle test without silent drops.
2. Emitted events from the server appear on all targeted clients in <200 ms under local conditions.
3. No browser console or server log shows CORS or version mismatch warnings.

Please document the changes you make and briefly explain any best-practice adjustments so I can maintain them going forward.