Event sourcing pattern fixes silent reconciliation drift in NestJS fintech backend

A NestJS-based fintech backend was experiencing recurring reconciliation drift, where internal ledger figures consistently failed to match payment processor records by small but significant amounts. Investigation revealed that three separate code paths — a webhook handler, a retry job, and an admin support tool — could each independently update a transaction's status without any shared record of changes. Because the system overwrote transaction states directly, there was no audit trail to determine whether a settlement had been applied once or multiple times. The fix involved replacing direct status updates with an event sourcing approach, where every status change is stored as a new immutable record that includes its origin source. This allows engineers to reconstruct a transaction's full history and quickly identify duplicate or conflicting updates during reconciliation reviews.