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>

tasks/models.py

@@ -188,7 +188,9 @@ class Task(models.Model):
 
         if self.due_time:
             due_moment = datetime.combine(self.due_date, self.due_time, tzinfo=user_tz)
-            return due_moment, due_moment
+            # Give "overdue" some breathing room after "due now" instead of
+            # firing both notifications at the exact same moment.
+            return due_moment, due_moment + timedelta(hours=1)
 
         overdue_moment = datetime.combine(self.due_date + timedelta(days=1), dt_time.min, tzinfo=user_tz)
         return None, overdue_moment