**feat(webhook):** add WEBHOOK_SECRET for token authentication support

- Protect `/webhook` endpoint using the `Authorization` header
- Update `README.md` with setup instructions and examples for authentication
- Warn when `WEBHOOK_SECRET` is not configured
- Add tests for valid, missing, and invalid token scenarios
- Update `docker-compose.yml` to support `WEBHOOK_SECRET` configuration

docker-compose.yml

@@ -4,4 +4,6 @@ services:
     build: .
     ports:
       - "8080:8080"
+    environment:
+      - WEBHOOK_SECRET=${WEBHOOK_SECRET:-}
     restart: unless-stopped