Release v0.4.0 - OOP Architecture Overhaul, Services & Repositories

🔍 Feature Enhancements

• **OOP Architecture Overhaul: Refactored core entities (Book, User, Purchase) to dataclasses in models.py. Introduced repository classes (BookRepository, UserRepository, PurchaseRepository) under bookstore_modules/db/ to encapsulate database queries and connection handling.
• **Dedicated Service Layer: Extracted domain operations into pure business services (AuthService, CatalogService, PurchaseService, AdminService, SearchLogRepository) under bookstore_modules/services/ free of console I/O side effects.
• **Decoupled User Interface: Isolated all input() and print() calls into dedicated CLI presentation components (ConsoleApp, UserMenu, AdminMenu) under bookstore_modules/user_interface/.

🔒 Security & Access Control

• **Restricted File Ingestion: Moved the bulk book import feature (import_from_file) from the unauthenticated public menu to AdminService / AdminMenu, ensuring bulk database writes require administrative privileges.

⚠️ Breaking Changes

• **Exception-Driven Business Logic: Replaced console print() error statements inside business workflows with domain exceptions inheriting from BookstoreError (UsernameTakenError, InsufficientStockError, BookNotFoundError, etc.).
• **Refactored Entry Point: Updated main.py to instantiate repositories, services, and ConsoleApp via dependency injection instead of invoking top-level procedural functions.

Full Changelog: v0.3.1...v0.4.0