Implements core improvements from Phase 1 of the Beancount patterns adoption:
- Uses Decimal for fiat amounts to prevent floating point errors
- Adds a meta field to journal entries for a full audit trail
- Adds a flag field to journal entries for transaction status
- Migrates existing account names to a hierarchical format
This commit introduces a database migration to add the `flag` and `meta` columns to the `journal_entries` table. It also includes updates to the models, CRUD operations, and API endpoints to handle the new fields.
