Migrates balance calculation and inventory tracking to
Fava/Beancount, leveraging Fava's query API for all
accounting calculations. This simplifies the core module
and centralizes accounting logic in Fava.
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`.
