[Change Requests] - Schedule the work-package-deadlines cron

Description

Context

A GitHub Actions workflow triggers the existing daily notification by curling /notifications/task-deadlines on a cron with the shared secret. We need the same for the new work-package-deadlines endpoint. The time should be 5 pm eastern

Task

Add a GitHub Actions cron workflow that calls /notifications/work-package-deadlines with the notification secret, at the UTC time corresponding to 5pm US Eastern.

Acceptance Criteria

• A workflow curls the new endpoint with Authorization: ${{ secrets.NOTIFICATION_ENDPOINT_SECRET }} against ${{ secrets.BACKEND_URL }}, mirroring the existing task-deadlines workflow.
• The cron is set to the UTC time for 5pm US Eastern, and the daylight-saving approach is written down: a single fixed UTC time accepted as "5pm-ish year-round," or two seasonal entries / a documented adjustment.
• Uses the same notifications environment and secrets as the existing workflow.

Start by locating the existing GitHub Actions workflow that curls /notifications/task-deadlines and review its notifications environment, backend URL, and notification secret usage. Add the matching work-package-deadlines workflow, set the UTC schedule for 5pm US Eastern with the daylight-saving choice documented, and confirm the endpoint, authorization, environment, and secrets match the acceptance criteria.