Extend update-metadata thumbnailUrl to Facebook (post-publish cover)

POST /v1/posts/{postId}/update-metadata already accepts thumbnailUrl, but its

platform enum is YouTube-only. Please extend it to Facebook.

Why it is small: the field exists, the endpoint exists, and Zernio already holds

the Page token. Meta exposes POST /{video_id}/thumbnails with source and

is_preferred for exactly this, and their Reels guide points to it for adding a

cover photo to a reel. So it is a routing change rather than new plumbing.

Use case: we publish 9:16 news video to five accounts and generate a designed

cover for each. mediaItems[].thumbnail handles it beautifully at publish time

(thank you). But videos published before we started sending a cover cannot be

fixed through the API at all — edit is text-only, PUT only updates recycling

config — so a back-catalogue stays on an auto-picked frame forever.

Requested: platform: "facebook" accepted on update-metadata with thumbnailUrl.