Admin: all-users job monitor and online activity

Site administrators can now monitor all annotation jobs across every user from a single view at Admin → All Jobs. Jobs are filterable by status (Pending / Running / Done / Failed) and by username, and all columns are sortable.

The User Management page now shows which users are currently online (active in the last 15 minutes) with a live green indicator, plus a relative "last seen" timestamp for recently active users. The admin dropdown also shows a quick count of online users.