Write a native post per network, without publishing

Returns one draft per requested network. Feed drafts straight back into /api/post as per_platform to publish exactly what was written. A draft may carry thread (X, Bluesky, Mastodon: when the idea asks for a thread; Bluesky and Mastodon also when it is too long for one post) and first_comment (LinkedIn, X, Bluesky, Mastodon: the idea's link, out of the body). platforms may be an array or a comma-separated string ("tiktok, instagram"); the idea may be sent as prompt, idea, text or topic. With no platforms, the account's connected networks are used. 400 codes: prompt_required, platforms_required, unknown_platform; 502 ai_failed when the writer fails. When the idea asks for a thread ("write a thread about…") and x is one of the networks, the X draft also has thread: ["…"] (the posts after text), ready for per_platform.x.thread; otherwise X gets one post. Same single AI call either way.