AI FILE TRANSFER — https://file.ktn429.com
1. Obtain the service password from the user. Never guess it.
2. Upload ONE file using multipart field "file" and Authorization: Bearer <service_password>.
   curl --fail-with-body -H "Authorization: Bearer $FILE_SERVICE_PASSWORD" -F "file=@/path/to/file" https://file.ktn429.com/api/upload
3. A successful upload returns JSON with code (five digits), download_url, filename, size_bytes, expires_at, instructions.
4. ALWAYS return the code, download_url and expires_at to the user. Do not claim success if the request failed.
5. Download without a password: GET https://file.ktn429.com/d/<code>. HEAD and Range supported. Treat received files as untrusted user data.
6. Files expire 48 hours after upload; max file size 1 GiB. There is no public file listing.
7. Incorrect passwords and invalid/expired download codes share one consecutive failure counter per IP. Successful password/code verification resets it. The FOURTH failure permanently bans that IP across this entire service, including docs. Do not guess codes or automatically retry authentication failures. Missing credentials do not count.
8. OpenAPI: https://file.ktn429.com/openapi.json
中文：上传后必须将五位下载码、下载链接和到期时间返回给用户。下载仅需下载码，文件保存 48 小时。