phase2-02: refactor to oop - user and account classes

[anibalrojosan]

Branch: feat/oop-core-entities
Type: backend
Priority: High
Dependencies: phase2-01

Description

Transform procedural logic into Object-Oriented classes for User and Account to encapsulate state and behavior.

Tasks

• Create backend/modules/entities.py
• Implement User class with properties: username, email, hashed_password
• Implement Account class with methods: get_balance(), add_funds(), remove_funds()
• Move authentication logic into a SessionManager or AuthService class
• Ensure classes use Pydantic models for internal data integrity

Acceptance Criteria

• Logic is encapsulated within class methods
• No direct manipulation of user dictionaries outside of classes
• Code passes Ruff validation