Pinterest video pins failed with "provide cover_image_url ... or
cover_image_key_frame_time" because no cover was sent. Default to
cover_image_key_frame_time 0 (first frame) when the user provides no
cover_image_url — verified against Pinterest's OpenAPI spec (integer
seconds, minimum 0, falls back to the last frame past the duration).
YouTube Shorts derive their required title from the post content, so the
publisher rejects an empty post — but the editor still let it be scheduled.
Gate it in compliance: a youtube_short with empty content is now flagged
(requires_text) so it can't be scheduled, instead of failing at publish.
27151bd925