Docs & example: SSE live updates via native EventSource API

[mkjsix]

Objective

Add a working SSE example to the product-catalog demo and document the pattern in the developer guide. This implements the documentation track defined in #6.

Prerequisite

• HtmlResponseInterceptor bypasses Accept: text/event-stream requests (Core hardening: bypass HTML interceptor for SSE event-stream requests #8 ✅)

Developer guide

Add a dedicated SSE section to docs/DEVELOPERS_GUIDE.md. Content defined in #6:

• EventSource API basics (connect, listen, close, auto-reconnect)
• RESTHeart SSE endpoint format (/_streams/<name>)
• Full working example using EventSource + htmx.ajax() for re-rendering
• When to use SSE vs polling
• Auth considerations
• Guidance on high-frequency flows (client-side rendering, not SSR)

Product-catalog example

Wire one live update scenario in examples/product-catalog:

• Product list (/shop/products) auto-refreshes when a product is inserted or deleted
• Uses native EventSource connected to the RESTHeart change stream for the products collection
• On event: calls htmx.ajax() to re-fetch and re-render the product list fragment
• Demonstrates at least one visible live DOM update without a page reload

Update examples/product-catalog/README.md with:

• Prerequisites (MongoDB replica set required for change streams)
• How to trigger a visible update (e.g. insert a document via curl or the admin UI)

Acceptance criteria

• SSE section added to developer guide
• EventSource pattern documented with a complete working example
• Auth and reconnect behaviour documented
• High-frequency flows guidance included
• Product-catalog example shows a live update
• README run instructions are reproducible