Add CI badge to README

Added CI badge to README for build status.

Author: maesse

README.md

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 A tiny debug HTTP interceptor: it exposes an `/inbound` endpoint that accepts any POST request, stores it in memory, and shows it in a minimalist Vue UI. Includes a FastAPI backend API for listing, viewing, and deleting captured requests.
 
+[![CI](https://github.com/maesse/HTTPIntercepter/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/maesse/HTTPIntercepter/actions/workflows/ci.yml)
+
 ## Structure
 
 - `backend/` — FastAPI app, pytest tests