Post-publish Reel cover for Instagram — if Meta permits

Sibling to the Facebook request: allow thumbnailUrl on

POST /v1/posts/{postId}/update-metadata for Instagram as well.

Flagging honestly that Meta is the constraint here, not Zernio — IG's Graph API

is much stricter about changing a Reel cover after publishing than Facebook is.

So this is "if Meta permits": whoever scopes it should check whether a cover

change is possible post-publish at all before any work is done.

If it is not possible, saying so in the docs would itself be valuable — right now

instagramThumbnail reads as a publish-time-only field without stating that it

can never be changed afterwards, which is the thing people will assume.