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/admin.py

@@ -21,8 +21,8 @@ class NotificationAdmin(admin.ModelAdmin):
 class ScheduledReminderAdmin(admin.ModelAdmin):
     """Admin for ScheduledReminder."""
 
-    list_display = ['task', 'remind_at', 'is_sent', 'sent_at', 'created_at']
-    list_filter = ['is_sent', 'remind_at']
+    list_display = ['task', 'reminder_type', 'remind_at', 'is_sent', 'sent_at', 'created_at']
+    list_filter = ['reminder_type', 'is_sent', 'remind_at']
     search_fields = ['task__title']
     ordering = ['remind_at']
     raw_id_fields = ['task']