Support Chat API

Server is running. This is an API backend — open the chat in the Flutter app (make run-flutter), not in the browser.

• /health — health check
• POST /chat/message — send a message
• GET /chat/history — message history
• POST /chat/upload — send an image (multipart)
• GET /chat/files/… — uploaded images
• GET /chat/ws?token=… — WebSocket for live replies