Fix daily email timing to work across all timezones

Critical fix: Task now runs every hour instead of once at 6 AM UTC.
This ensures users in all timezones receive their email at 6 AM local time.

Changes:
- Run task every hour instead of once daily
- Check if it's 6-7 AM in user's timezone (1 hour window)
- Track sent emails in Notification model to prevent duplicates
- Add 'daily_email' notification type

Without this fix, users in timezones where 6 AM UTC is not morning
would never receive their daily email.

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

notifications/models.py

@@ -17,6 +17,7 @@ class Notification(models.Model):
         ('overdue', 'Overdue'),
         ('shared', 'Task Shared'),
         ('comment', 'Comment'),
+        ('daily_email', 'Daily Email'),
     ]
 
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)