Event Sourcing over CRUD for Audit Trails

Decision: Replace standard CRUD in PostgreSQL with Event Sourcing for financial transaction audit trails.

Context: Compliance required history of transaction #8821 (who changed status from pending to approved). Standard CRUD overwrites previous states on UPDATE, losing business context. Database triggers were considered but rejected because they only capture row-level changes (e.g., status 1 to 2) without knowing why the change happened or user intent.

Solution: Store immutable domain events (TransactionCreated, TransactionApproved, StatusUpdated) in Kafka as the event log. Build a materialized view in Postgres for fast frontend reads. This satisfies compliance by preserving full audit context.

Files involved:
- src/services/transactionService.ts (event producers)
- src/events/eventStore.ts (consumer/store logic)

Reference: Martin Fowler's "Event Sourcing Pattern" article.

Alternatives rejected:
1. Standard CRUD - loses history on UPDATE
2. Database triggers - loses business context, only sees row changes