POST /api/v1/posts takes multipart for TikTok only, so a YouTube file upload
has its own route. Fields use the same names; booleans are the strings
"true" or "false", and tags is comma-separated. Send one video file of
up to 4 GiB and, optionally, a thumbnail (JPEG or PNG, up to 50 MB).
YouTube allows custom thumbnails only on verified channels; a refused thumbnail
is a warning, not a failed upload.
It returns 202 once YouTube has the file, with the same response as
Publish to YouTube. Processing continues
after; poll YouTube upload status.