Schedule different variations across accounts

Spreads DIFFERENT variations of one cloning job across the brand's social accounts at staggered times. Every account gets its own variation and the same variation is never posted to two accounts: a variation that already has a live or published post is refused (409) unless the entry is identical to it, in which case it is reported as alreadyScheduled and nothing is sent again. Surplus variations go out on the next day(s), one per account per day, and an account-day that already has a post from this job (an earlier distribute, a retry after a partial failure) is skipped — an entry landing on one is refused with 409 unless it is that identical post. Originals are handed out before translated variations; languages narrows a server plan to some of them. Send either an explicit plan, or socialAccountIds + startAt (+ spacingMinutes, jitterMinutes, languages) for the server to plan with the same rules the dashboard previews (seeded on the job, UTC days unless timeZone is given); accounts that cannot take any variation (a carousel to YouTube, a locked or disconnected account) are returned under excluded. The whole plan is validated before anything is scheduled — ownership, the job's brand (a plan-locked brand is a 402), rendering, platform compatibility, duplicates, one post per account per day, the time window and the monthly video quota. One distribute per job runs at a time (409 in_progress). Each post uses the variation's own caption and hashtags. A dispatch failure after validation is reported per entry. Requires publish:write and an Idempotency-Key.