Event Sourcing over CRUD for Financial Transaction Audit Trails

Decision: Adopt Event Sourcing pattern instead of standard CRUD for financial transaction audit trails.

WHY: The current PostgreSQL CRUD approach overwrites previous states on every update. During a compliance audit for transaction #8821, the team could not determine who changed the status from pending to approved because update history was lost.

ALTERNATIVES CONSIDERED:
1. Database triggers writing to a history_logs table — REJECTED because triggers only see row-level changes (e.g., status 1→2) and lose business context and user intent behind the change.
2. Staying with CRUD — REJECTED because it inherently cannot provide full audit history.

IMPLEMENTATION:
- Immutable domain events stored via Kafka (e.g., TransactionCreated, TransactionApproved, StatusUpdated)
- Materialized views in PostgreSQL for fast frontend reads
- Event producers in src/services/transactionService.ts
- Event consumer/store logic in src/events/eventStore.ts

REFERENCE: Martin Fowler's 'Event Sourcing Pattern' article — provides the industry-standard reference architecture for this approach.

This satisfies compliance requirements by preserving complete event history with full context.