Implementation Plan: Cron-based cleanup of stale RTC post meta

Introduce a cron task or other mechanism to clean up sync messages left behind by the real-time collaboration default provider, which are stored in post meta against a special post type.

Approach

• A daily WP-Cron event (wp_sync_cleanup) queries for sync updates older than n days.
• Each sync update value is an associative array envelope with a timestamp property.
• If the sync update older than the expiration period, the post meta is deleted.
• The expiration defaults to 7 days and is configurable via the wp_sync_cleanup_expiration filter.