Starts a file upload. Returns a pre-signed presignUrl and a uuid that identifies this upload.

Upload flow:

1. Call this endpoint to receive presignUrl and uuid.
2. HTTP PUT the raw file bytes to presignUrl (the file itself is the request body; do not send session cookies or API auth headers - the URL is self-authorizing). The URL is valid for 24 hours.
3. Call GET /rest/manager/files/getpresignfinal/{uuid} to finalize the upload and obtain the permanent download URL.

Example with curl: curl -X PUT --upload-file myapp.apk "<presignUrl>"