Event Sourcing over CRUD for Financial Transaction Audit Trails

DECISION: Move from standard CRUD in PostgreSQL to Event Sourcing for financial transactions.

PROBLEM: Standard CRUD overwrites previous states. When compliance requested the history of transaction #8821, the team could not provide who changed the status from pending to approved.

ALTERNATIVES CONSIDERED:
1. Database triggers on a history_logs table — REJECTED because business context is lost. A trigger only sees row changes (e.g., status changed from 1 to 2), it doesn't know why the change happened or the user intent behind it.

CHOSEN APPROACH: Event Sourcing with Kafka. Store a sequence of immutable domain events (TransactionCreated, TransactionApproved, StatusUpdated) in an immutable event log via Kafka. Build a materialized view in PostgreSQL for fast reads on the frontend.

FILES AFFECTED:
- src/services/transactionService.ts — event producers
- src/events/eventStore.ts — consumer and store logic

REFERENCE: Martin Fowler's 'Event Sourcing Pattern' article.