Presign an X (Twitter) video upload

Requires scope: `publish:write`. Returns a presigned URL to PUT a video (max 500 MB, `video/mp4`) directly to storage, plus the `r2_key` to reference afterward. At publish time the video is uploaded to X via the v2 chunked flow (`media_category=tweet_video`); X enforces a 140-second duration cap.

curl -X GET "https://example.com/twitter/uploads/presign?content_type=string&file_size=0"

{  "upload_url": "https://r2.example.com/put?sig=...",  "r2_key": "twitter/4021/uuid.mp4"}