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/migrations/0004_scheduledreminder_reminder_type_and_more.py

@@ -0,0 +1,23 @@
+# Generated by Django 5.2.9 on 2026-09-05 15:41
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('notifications', '0003_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='scheduledreminder',
+            name='reminder_type',
+            field=models.CharField(choices=[('reminder', 'Before Due'), ('due_soon', 'Due Now'), ('overdue', 'Overdue')], default='reminder', max_length=20),
+        ),
+        migrations.AlterField(
+            model_name='notification',
+            name='notification_type',
+            field=models.CharField(choices=[('reminder', 'Task Reminder'), ('due_soon', 'Due Soon'), ('overdue', 'Overdue'), ('shared', 'Task Shared'), ('comment', 'Comment'), ('daily_email', 'Daily Email')], max_length=20),
+        ),
+    ]