Add node-lambda-sse streaming demo with React frontend

[jbernalvallejo]

This PR adds a new example to the apigw-response-streaming collection demonstrating Server-Sent Events (SSE) over API Gateway response streaming.

What's included

• Lambda function using streamifyResponse to stream Bedrock ConverseStream (Nova Lite) responses as SSE events
• React frontend (Vite) with TTFB tracking and a collapsible SSE event log for inspecting chunk timing and shape
• Terraform IaC with configurable AWS profile/region via terraform.tfvars
• Updated parent README with the new demo entry

Architecture

Client → API Gateway (STREAM) → Lambda → Bedrock ConverseStream → SSE events back to client

Testing

• Verified SSE streaming via both curl and the React frontend
• Confirmed terraform destroy cleans up all resources