Support monthly debit-credit journal entries in Core API

🔍 Problem Statement

The Core API does not support generating monthly debit-credit journal entries. Finance teams that need journal entries for their general ledger must manually calculate them from payment and invoice data, which is time-consuming and error-prone.

💡 User Story

As a finance team member or developer, I want monthly debit-credit journal entries available via the Core API, so that I can automate general ledger posting without manual calculations.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a Pelcro account with transaction activity, when a developer requests journal entries for a month via the API, then the response includes properly structured debit-credit entries that can be posted to a general ledger.