Fix celery container health issues

- Fix import error in notifications/tasks.py (timezone.datetime -> datetime)
- Add healthchecks to celery-worker and celery-beat containers
- Add procps package to Dockerfile for pgrep command
- Add email environment variables to celery containers

The import error was causing celery workers to crash when loading tasks.
Missing healthchecks prevented proper container health monitoring.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

notifications/tasks.py

@@ -226,7 +226,7 @@ def schedule_task_reminder(task_id):
         else:
             # Default to 9 AM on due date
             due_datetime = timezone.make_aware(
-                timezone.datetime.combine(task.due_date, timezone.datetime.min.time())
+                datetime.combine(task.due_date, datetime.min.time())
             ).replace(hour=9)
 
         remind_at = due_datetime - timedelta(minutes=reminder_minutes)