Retry the failed platforms of a failed or partially failed post

Re-publishes ONLY the platforms that failed, on the same post — platforms that already succeeded are never re-published. Works on posts with status failed (every platform failed) or warning (some platforms failed). The retry is processed asynchronously by the publishing queue, usually within a minute; poll GET /posts/{id} to see the outcome (status becomes published and published_urls gains the platform URL on success, or the platform's entry reappears in errors on another failure). Each platform can be retried at most 3 times; after that, create a new post. Platforms dismissed with 'ignore' in the dashboard are not retried.