- Add User model with email-based authentication
- Add Tag model with many-to-many relationship to todos
- Add TodoTag junction table for todo-tag relationships
- Enhance Todo model with priority levels (low, medium, high)
- Add due_date field with datetime support
- Add recurrence_pattern field for recurring todos
- Add parent-child relationship for subtasks support
- Create comprehensive alembic migration for all changes
- Add proper indexes for performance optimization
- Use Text type for todo descriptions
- Implement proper SQLAlchemy relationships and foreign keys
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