AVideo's IDOR in uploadPoster.php Allows Any Authenticated User to Overwrite Scheduled Live Stream Posters and Trigger False Socket Notifications

WWBN AVideo is an open source video platform. In versions up to and including 26.0, the `plugin/Live/uploadPoster.php` endpoint allows any authenticated user to overwrite the poster image for any scheduled live stream by supplying an arbitrary `live_schedule_id`. The endpoint only checks `User::isLogged()` but never verifies that the authenticated user owns the targeted schedule. After overwriting the poster, the endpoint broadcasts a `socketLiveOFFCallback` notification containing the victim's broadcast key and user ID to all connected WebSocket clients. Commit 5fcb3bdf59f26d65e203cfbc8a685356ba300b60 fixes the issue.