notification_cursors table stores one read-cursor per user per tenant;
GET /v1/activity returns unreadCount + readAt, POST /v1/activity/read
persists the cursor. Reminder dispatches now append reminder.sent /
reminder.failed system events so delivery history reaches the feed.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
