security: Implement a "Maintenance Mode" middleware to block requests

[AlAfiz]

Description

If we need to perform an emergency database migration or smart contract upgrade, we cannot have users executing trades simultaneously.
We need a global "Maintenance Mode" middleware that instantly blocks all API traffic except for admin monitoring routes.
When activated, every standard route should safely return a 503 Service Unavailable with a friendly message.
This prevents data corruption during critical Wave 3 deployments.

Requirements

• Add a MAINTENANCE_MODE=false variable to the .env file.
• Create a custom middleware function that checks this variable.
• If true, return { success: false, message: "TradeFlow is currently undergoing maintenance. Please try again later." }.
• Apply this middleware globally, but ensure the /ping or /status endpoints bypass it.