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>

notifications/tests.py

@@ -41,7 +41,7 @@ class RescheduleRemindersTests(TestCase):
 
         expected_due = django_timezone.make_aware(datetime(2026, 1, 15, 17, 0, 0))
         self.assertEqual(due_soon.remind_at, expected_due)
-        self.assertEqual(overdue.remind_at, expected_due)
+        self.assertEqual(overdue.remind_at, expected_due + timedelta(hours=1))
         self.assertEqual(before_due.remind_at, expected_due - timedelta(minutes=30))
 
     def test_date_only_task_only_gets_overdue_reminder_next_day(self):
@@ -107,6 +107,72 @@ class RescheduleRemindersTests(TestCase):
         self.assertEqual(ScheduledReminder.objects.filter(task=task, is_sent=False).count(), 3)
 
 
+class DefaultReminderMinutesChangeTests(TestCase):
+    """
+    Task.reschedule_reminders() only captures user.default_reminder_minutes
+    at the moment a task's own due_date/due_time is set - changing the
+    profile setting afterward doesn't reach existing tasks on its own.
+    User.reschedule_reminder_notifications(), triggered from User.save()
+    when the setting changes, is what makes that retroactive.
+    """
+
+    def setUp(self):
+        self.user = User.objects.create_user(
+            username='reminderuser2', email='reminderuser2@example.com', password='testpass123',
+            default_reminder_minutes=30,
+        )
+
+    def test_changing_default_minutes_reschedules_before_due_reminder(self):
+        task = Task.objects.create(
+            user=self.user, title='Task', due_date=date(2026, 1, 15), due_time=dt_time(17, 0, 0),
+        )
+        due_moment = django_timezone.make_aware(datetime(2026, 1, 15, 17, 0, 0))
+        before_due = ScheduledReminder.objects.get(task=task, reminder_type='reminder')
+        self.assertEqual(before_due.remind_at, due_moment - timedelta(minutes=30))
+
+        self.user.default_reminder_minutes = 15
+        self.user.save()
+
+        before_due = ScheduledReminder.objects.get(task=task, reminder_type='reminder')
+        self.assertEqual(before_due.remind_at, due_moment - timedelta(minutes=15))
+
+    def test_task_with_explicit_reminder_at_is_left_alone(self):
+        custom_reminder = django_timezone.make_aware(datetime(2026, 1, 15, 9, 0, 0))
+        task = Task.objects.create(
+            user=self.user, title='Task', due_date=date(2026, 1, 15), due_time=dt_time(17, 0, 0),
+            reminder_at=custom_reminder,
+        )
+
+        self.user.default_reminder_minutes = 15
+        self.user.save()
+
+        before_due = ScheduledReminder.objects.get(task=task, reminder_type='reminder')
+        self.assertEqual(before_due.remind_at, custom_reminder)
+
+    def test_completed_task_is_not_touched(self):
+        task = Task.objects.create(
+            user=self.user, title='Task', due_date=date(2026, 1, 15), due_time=dt_time(17, 0, 0), status='completed',
+        )
+        self.assertFalse(ScheduledReminder.objects.filter(task=task, is_sent=False).exists())
+
+        self.user.default_reminder_minutes = 15
+        self.user.save()
+
+        self.assertFalse(ScheduledReminder.objects.filter(task=task, is_sent=False).exists())
+
+    def test_unrelated_profile_field_save_does_not_reschedule(self):
+        task = Task.objects.create(
+            user=self.user, title='Task', due_date=date(2026, 1, 15), due_time=dt_time(17, 0, 0),
+        )
+        original_id = ScheduledReminder.objects.get(task=task, reminder_type='reminder').id
+
+        self.user.first_name = 'Changed'
+        self.user.save()
+
+        # Same row (not deleted and recreated by reschedule_reminders()).
+        self.assertEqual(ScheduledReminder.objects.get(task=task, reminder_type='reminder').id, original_id)
+
+
 class SendScheduledRemindersTests(TestCase):
     """Tests for the send_scheduled_reminders Celery task."""
 
@@ -131,11 +197,30 @@ class SendScheduledRemindersTests(TestCase):
             mock_now.return_value = reminder.remind_at + timedelta(seconds=1)
             sent_count = send_scheduled_reminders()
 
-        self.assertEqual(sent_count, 3)  # reminder, due_soon, overdue all past remind_at by then
-        self.assertEqual(len(mail.outbox), 3)
+        # "reminder" (30 min before) and "due_soon" have passed; "overdue"
+        # is delayed an hour past due_soon so it hasn't fired yet.
+        self.assertEqual(sent_count, 2)
+        self.assertEqual(len(mail.outbox), 2)
         reminder.refresh_from_db()
         self.assertTrue(reminder.is_sent)
         self.assertIsNotNone(reminder.sent_at)
+        self.assertFalse(ScheduledReminder.objects.get(task=task, reminder_type='overdue').is_sent)
+
+    def test_overdue_reminder_fires_an_hour_after_due(self):
+        task = Task.objects.create(
+            user=self.user, title='Water the plants', due_date=date(2026, 1, 15), due_time=dt_time(17, 0, 0),
+        )
+        overdue = ScheduledReminder.objects.get(task=task, reminder_type='overdue')
+        due_soon = ScheduledReminder.objects.get(task=task, reminder_type='due_soon')
+        self.assertEqual(overdue.remind_at, due_soon.remind_at + timedelta(hours=1))
+
+        with patch('django.utils.timezone.now') as mock_now:
+            mock_now.return_value = overdue.remind_at + timedelta(seconds=1)
+            send_scheduled_reminders()
+
+        overdue.refresh_from_db()
+        self.assertTrue(overdue.is_sent)
+        self.assertIn(f'Overdue: {task.title}', [m.subject for m in mail.outbox])
 
     def test_future_reminder_not_sent_yet(self):
         task = Task.objects.create(