Feat: Comprehensive API, Tag Management, and Web UI Improvements

[JmRILKayn]

This Pull Request introduces significant enhancements to the Flask To-Do application, focusing on improved data organization, expanded functionality through a REST API, and refined user experience.

Key Features and Improvements:

• Full REST API: Implemented complete CRUD operations for both To-Do items and Tags, including robust error handling and status codes.
• Tag Management: Introduced a many-to-many relationship allowing multiple tags per To-Do, with dedicated Tag CRUD functionality.
• Enhanced Web UI: Integrated tag display, filtering, and in-place editing for To-Dos, along with improved input validation.
• Comprehensive Testing: Added extensive unit tests for all new features and refactored routes, achieving high test coverage.