Internal endpoint called by the Celery Beat periodic task to sync new Zotero annotations for all users whose items haven't been synced in the past 24 hours. Auth: shared secret via Authorization header (JOBS_INTERNAL_SECRET env var).

Dispatch-and-return: syncing every due user can take minutes — far past the load balancer's timeout — so the response only reports how many users were dispatched. The per-user work runs as a background task, and its outcomes (per-user errors, final summary) are logged here on the server.