Delay overdue notification an hour, retroactively apply reminder setting

Two refinements from live testing of the reminder feature:

- Due-time tasks scheduled "due now" and "overdue" at the exact same
  moment. Task._due_and_overdue_moments() now delays "overdue" by an
  hour so they don't arrive together. The overdue badge/styling
  elsewhere is unaffected - only this notification's timing changes.

- Task.reschedule_reminders() only captures user.default_reminder_minutes
  at the moment a task's own due_date/due_time is set, so changing the
  profile setting didn't reach tasks whose due date was already set.
  User.save() now detects a change to that setting and calls the new
  User.reschedule_reminder_notifications(), which recomputes the
  "before due" reminder on active due tasks that don't have their own
  explicit reminder_at override.

8 new/updated tests cover the overdue delay and the retroactive
rescheduling (including that unrelated profile saves and tasks with an
explicit reminder_at are left untouched).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

users/models.py

@@ -57,6 +57,39 @@ class User(AbstractUser):
     def __str__(self):
         return self.email
 
+    @classmethod
+    def from_db(cls, db, field_names, values):
+        instance = super().from_db(db, field_names, values)
+        instance._loaded_default_reminder_minutes = dict(zip(field_names, values)).get('default_reminder_minutes')
+        return instance
+
+    def save(self, *args, **kwargs):
+        loaded_reminder_minutes = getattr(self, '_loaded_default_reminder_minutes', None)
+        reminder_minutes_changed = (
+            loaded_reminder_minutes is not None and loaded_reminder_minutes != self.default_reminder_minutes
+        )
+
+        super().save(*args, **kwargs)
+        self._loaded_default_reminder_minutes = self.default_reminder_minutes
+
+        if reminder_minutes_changed:
+            self.reschedule_reminder_notifications()
+
+    def reschedule_reminder_notifications(self):
+        """
+        Recompute "before due" reminders for active tasks using the current
+        default_reminder_minutes. Task.reschedule_reminders() only captures
+        this setting at the moment a task's own due_date/due_time is set,
+        so it doesn't apply retroactively on its own - this is what makes a
+        profile-level change to the setting reach existing tasks. Tasks
+        with their own explicit reminder_at override are left alone.
+        """
+        tasks = self.tasks.filter(
+            due_date__isnull=False, is_deleted=False, reminder_at__isnull=True,
+        ).exclude(status__in=('completed', 'cancelled'))
+        for task in tasks:
+            task.reschedule_reminders()
+
 
 class DeviceToken(models.Model):
     """