fix: implement graceful shutdown for Express server

[ParthBhuptani]

📋 What does this PR do?

Updates server/index.js to handle SIGTERM and SIGINT signals
gracefully instead of killing the process instantly.

When a shutdown signal is received:

1. Stops accepting new incoming requests (server.close())
2. Waits for existing in-flight HTTP requests to finish
3. Cleanly closes the MongoDB connection (mongoose.connection.close())
4. Exits the process cleanly (process.exit(0))

🔗 Related Issue

Closes #940

🛠️ Changes Made

• server/index.js — stored server instance in a variable and added
  graceful shutdown handler for SIGTERM and SIGINT signals

✅ Benefits

• Prevents data corruption during deployments
• Prevents users getting abrupt 502 errors during server restarts
• Required for zero-downtime deployments and Docker scaling

✅ Checklist

• I've read the CONTRIBUTING guide
• My code follows the project's style guidelines
• I've linked the related issue
• I haven't introduced any new secrets or API keys