TikTok Custom Cover Image URL

Title: TikTok: Custom Cover Image URL (not just frame selection)

Body:

Right now TikTok only lets you pick a frame from the video via video_cover_timestamp_ms. Would be really helpful to also support a custom cover image URL—like how YouTube thumbnails work.

Why this would help:

• I spend time making good thumbnails for YouTube—let me use them on TikTok too

• Custom covers perform way better than auto-selected frames

• Keeps my branding consistent everywhere

Suggested API Parameter:

json

{  "tiktokSettings": {  "cover_url": "https://example.com/thumbnail.jpg",  "video_cover_timestamp_ms": 3000 // fallback if cover_url isn't supported  } }

Totally understand if TikTok's API doesn't support this—if that's the case, maybe just note it as a platform limitation in the docs. But if it does, this would be huge!