Reply Preview Case

The conversation the reply preview answers — the same case resolution as POST /reply-preview minus the LLM, so the Replies tab shows the thread the moment it opens while the draft generates (T-1906, step 2a of the T-1289 rollout). Real replies only: a project (or pinned person_id) without a real inbound reply gets case: null — never a synthetic inbound (the tab renders its own empty state). Deliberately uncached, like /reply-preview.