phase2-07: implement unit tests for financial logic

[anibalrojosan]

Branch: feat/unit-tests-phase2
Type: quality
Priority: High
Dependencies: phase2-03

Description

Create a comprehensive suite of unit tests using Pytest to validate the new OOP logic and transaction safety.

Tasks

• Create backend/tests/test_entities.py
• Test Account balance operations
• Test TransactionManager with successful transfers
• Test edge cases: insufficient funds, self-transfer, negative amounts
• Mock database calls for unit testing logic

Acceptance Criteria

• 100% coverage of core financial logic
• All tests pass with uv run pytest
• Tests are descriptive and follow best practices