Add per-task reminder notifications: before due, due now, and overdue

Only proactive notification was the once-a-day 6-7AM digest. Wires up
the previously-unused User.default_reminder_minutes profile setting
and ScheduledReminder model (Gitea #8) to send timely per-task alerts:

- Task.reschedule_reminders(), hooked into Task.save() via a dirty-check
  against due_date/due_time/reminder_at/status/is_deleted, (re)creates up
  to 3 ScheduledReminder rows per active due task: a "before due" reminder
  (from reminder_at if set, else default_reminder_minutes before due), a
  "due now" notification, and an "overdue" notification (mirroring
  is_overdue's day-after rule for date-only due tasks). Hooking into
  save() means every call site - web views, the API, and sync - picks
  this up automatically.
- New Celery task send_scheduled_reminders (beat schedule: every 5 min)
  sends due reminders via whichever of email/push the user has enabled,
  reusing the existing per-user channel toggles, and logs them to the
  Notification table.
- 14 new tests covering the scheduling math, due-date-change/completion/
  deletion cleanup, and the sending task's channel and timing behavior.

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

notifications/models.py

@@ -50,14 +50,23 @@ class Notification(models.Model):
 
 class ScheduledReminder(models.Model):
     """
-    Tracks scheduled reminders for tasks.
+    Tracks scheduled reminders for tasks. Rows are (re)created by
+    Task.reschedule_reminders() whenever a task's due info changes, and
+    consumed by notifications.tasks.send_scheduled_reminders().
     """
+    REMINDER_TYPES = [
+        ('reminder', 'Before Due'),
+        ('due_soon', 'Due Now'),
+        ('overdue', 'Overdue'),
+    ]
+
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
     task = models.ForeignKey(
         'tasks.Task',
         on_delete=models.CASCADE,
         related_name='scheduled_reminders'
     )
+    reminder_type = models.CharField(max_length=20, choices=REMINDER_TYPES, default='reminder')
     remind_at = models.DateTimeField()
     is_sent = models.BooleanField(default=False)
     sent_at = models.DateTimeField(null=True, blank=True)
@@ -68,4 +77,4 @@ class ScheduledReminder(models.Model):
         ordering = ['remind_at']
 
     def __str__(self):
-        return f"Reminder for {self.task.title} at {self.remind_at}"
+        return f"{self.get_reminder_type_display()} for {self.task.title} at {self.remind_at}"