Integrates Fava/Beancount for managing journal entries.
This change introduces functions to format entries into Beancount
format and submit them to a Fava instance.
It replaces the previous direct database entry creation with Fava
submission for expense, receivable, and revenue entries. The existing
create_journal_entry function is also updated to submit generic
journal entries to Fava.
Introduces utilities to format Castle data models into Beancount
transactions for Fava API compatibility.
Provides functions to format transactions, postings with cost basis,
expense entries, receivable entries, and payment entries.
These functions ensure data is correctly formatted for Fava's
add_entries API, including cost basis, flags, and metadata.
