feat: implement Server-Sent Events (SSE) infrastructure

[mrehandev]

Add complete SSE implementation with real-time server-to-client communication:

• Server-side SSE manager with client tracking and heartbeat mechanism
• API endpoints for SSE connections and event triggering
• React hook (useSSE) for client-side SSE handling
• Demo page with interactive SSE testing interface
• Support for multiple event types (test, broadcast, system updates)
• Automatic reconnection and connection lifecycle management
• Test utilities for SSE functionality validation

The implementation includes proper TypeScript types, error handling,
and cleanup mechanisms for production-ready real-time communication.

Key reasons for adding this package:

• Testing: Allows us to write automated tests for SSE endpoints using Node.js test runners
• Development: Enables CLI-based testing and debugging without needing a browser (Just like most of the today's SSE packages we have for helping in coding etc..)
• CI/CD: Facilitates integration testing in headless environments
• Monitoring: Can be used to build health check scripts for production SSE endpoints