feat: use celery instead of cron jobs for notification digest

[AhtishamShahid]

Description

This pull request implements a significant refactor of the email digest scheduling system for notifications. The main change is the shift from using a management command and cron jobs to an automated, deduplicated, and transactional Celery-based scheduling system for daily and weekly digest emails. This ensures that digest emails are reliably scheduled and sent without duplication, even when multiple notifications arrive concurrently. The update introduces a new DigestSchedule model to track scheduled digest tasks, and deprecates the old management command. Additionally, timezone handling has been improved to use Django's timezone utilities.

[Copilot]

Pull request overview

Refactors notification digest scheduling from cron/management-command driven execution to Celery-based, deduplicated per-user scheduling backed by a new DigestSchedule model and updated time-window handling.

Changes:

• Add configurable daily/weekly digest delivery schedule settings and a DigestSchedule model to dedupe scheduled digest tasks.
• Trigger per-user digest scheduling from send_notifications, and introduce a new delayed Celery task flow for digest delivery.
• Deprecate the legacy send_email_digest management command and adjust digest date-range calculation to use Django timezone utilities.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
openedx/envs/common.py	Adds instance-configurable UTC delivery schedule settings for daily/weekly digests.
openedx/core/djangoapps/notifications/tasks.py	Schedules digest delivery when notifications are created for daily/weekly cadence users.
openedx/core/djangoapps/notifications/models.py	Introduces DigestSchedule model used as the dedupe source of truth for scheduled digest tasks.
openedx/core/djangoapps/notifications/migrations/0012_digestschedule.py	Creates the DigestSchedule table and unique constraint.
openedx/core/djangoapps/notifications/management/commands/send_email_digest.py	Marks the cron-driven command as deprecated and changes runtime behavior.
openedx/core/djangoapps/notifications/email/utils.py	Switches digest window calculations to django.utils.timezone.now() and returns aware datetimes.
openedx/core/djangoapps/notifications/email/tasks.py	Implements new digest scheduling/deduping logic and a per-user delayed Celery task flow.
openedx/core/djangoapps/notifications/email/tests/test_tasks.py	Adds/updates tests for the new digest scheduling and delivery behavior.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.