Refactors the data model to use a single 'amount' field for journal entry lines, aligning with the Beancount approach.
This simplifies the model, enhances compatibility, and eliminates invalid states.
Includes a database migration to convert existing debit/credit columns to the new 'amount' field.
Updates balance calculation logic to utilize the new amount field for improved accuracy and efficiency.
Refactors the accounting logic into a clean, testable core module, separating business logic from database operations.
This improves code quality, maintainability, and testability by creating a dedicated `core/` module, implementing `CastleInventory` for position tracking, moving balance calculations to `core/balance.py`, and adding comprehensive validation in `core/validation.py`.
