Allow Editing Posts During Per-Channel Stagger

Currently, PUT /v1/posts/{id} is rejected while a post has status: "publishing".

With per-platform scheduledFor, a post can remain in publishing for several days while some platforms are already published and others are still pending. This effectively makes the post uneditable during the entire stagger window.

Request: Please allow editing a publishing post by modifying only the platform targets that are still pending, while leaving already published platforms untouched.