- Create migration 005 to add parent_id and level columns to todos tables
- Fix migration 003 to use correct table name (note_todos instead of todos)
- Update POST /api/notes/:id/todos to accept parent_id parameter
- Update POST /api/todos to accept parent_id for global todos
- Add automatic level calculation based on parent depth
- Update GET endpoints to return parent_id and level fields
- Add validation to ensure parent todo exists before creating subtask
- Support CASCADE DELETE for subtasks when parent is deleted
