Copy drafts (v2)

Supports original MP3 and WAV audio alongside the existing study media. Copy a Research campaign's saved Custom study into a new unfinished study draft of the signed-in user's: its material and questions as the campaign froze them, its business profile, its schedule (once, weekly or monthly) and the customers on its roster as the draft's picked customers (none when the roster holds more than a draft can pick). Send a new draft id as id and the campaign's id as campaignId; the copy is that draft at revision 1, which research_drafts_get_v3 reads and research_drafts_save_v3 edits. Launching it (research_campaigns_launch) starts a new campaign, and the copied campaign stays as it ran: a one-time study runs once, so a changed study is a new one. A study other than Custom is refused 409 PRECONDITION_FAILED (details.reason not_copyable); a deleted, unknown or other tenant's campaign answers 404. Drafts belong to the user who saves them, so an API key cannot copy one.