Add expandable multi-channel WebSocket architecture

[VrindaBansal]

Summary

This PR makes the WebSocket server expandable to support any number of job types through a channel-based architecture.

Changes

• Added CHANNELS array for easy expansion to new job types
• Auto-subscribe to all defined channels on startup
• Channel info included in all forwarded messages
• Support for multiple job types: job-updates, file-upload, bulk-upload, pdf-extraction

Architecture

The server now:

1. Subscribes to all channels defined in the CHANNELS array
2. Forwards messages with channel metadata to subscribed clients
3. Requires only 1 line change to add a new job type

Documentation

Added comprehensive EXPANDABLE_WEBSOCKET_GUIDE.md with:

• Architecture overview
• Step-by-step guide for adding new job types
• Examples and usage patterns
• Testing instructions

How to Add New Job Type

Simply add one line to the CHANNELS array:

const CHANNELS = [
  "job-updates",
  "file-upload",
  "bulk-upload",
  "pdf-extraction",
  "your-new-channel", // ← Add here
] as const;

No infrastructure changes needed!