GHSA-wch8-mhj5-9frg: Open WebUI: Cross-user file disclosure via /api/chat/completions image_url field

Open WebUI has a security flaw where authenticated users can access other users' files through the `/api/chat/completions` endpoint. When a user sends an image request with a file ID (instead of a web link) in the `image_url` field, the server reads that file from disk without checking if the user owns it, then converts it to a format the AI can process. An attacker can exploit this by using another user's file ID to make the AI read and describe private files, leaking their content.