GHSA-pg97-jvmf-qfvc: djust has Cross-Site Request Forgery on the Server-Sent-Events transport: a cross-origin page can drive a vict

### Impact The SSE client→server POST endpoints are `@csrf_exempt` and the SSE GET stream endpoint had no Origin check, so a cross-origin page could drive a victim-cookie-authenticated SSE session: force the victim's browser to GET the stream URL (which **creates and mounts** a LiveView as the victim) and POST to the message endpoint with `credentials: include` to **fire state-changing event handlers as the victim**. …