Event Sourcing over CRUD for Audit Trails

DECISION: Move from standard CRUD in PostgreSQL to Event Sourcing for financial transaction audit trails to ensure compliance.

PROBLEM: Standard CRUD updates overwrite previous states, so compliance requests cannot be satisfied. Example: when compliance asked for the history of transaction #8821, the team could not provide who changed the status from pending to approved.

ALTERNATIVES CONSIDERED AND REJECTED:
- Database triggers writing to a history_logs table: rejected because business context is lost. A trigger only sees row changes (e.g., status changed from 1 to 2) but does not know WHY the change happened or the user intent behind it.

APPROVED ARCHITECTURE:
- Store a sequence of immutable domain events (e.g., TransactionCreated, TransactionApproved, StatusUpdated) instead of mutable state.
- Use Kafka to store the immutable event log.
- Build a materialized view in PostgreSQL for fast reads on the frontend.

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

REFERENCE: Martin Fowler's 'Event Sourcing Pattern' article — followed as the reference architecture to avoid building blind; it satisfies the audit requirement and is an industry standard.