📝 Description
This PR implements a nightly automated task to manage invoice lifecycles. Using @nestjs/schedule, the system now scans for overdue "pending" invoices every day at 02:00 UTC and transitions them to an "expired" state.

🎯 Key Changes
Cron Implementation: Added InvoiceCronService utilizing CronExpression.EVERY_DAY_AT_2AM.

State Transition: Automated the update of pending → expired for all invoices where dueDate < now().

Event-Driven Architecture: Integrated EventEmitter2 to dispatch InvoiceExpiredEvent, allowing the notification service to trigger emails or webhooks without coupling.

Resilience: Wrapped the execution in a try-catch block with structured logging to ensure that database failures do not crash the application instance.

💻 Implementation Snippet (NestJS)
The core logic ensures idempotency by filtering specifically for pending statuses:

TypeScript
@Cron('0 0 2 * * *') // 02:00 UTC
async handleInvoiceExpiration() {
this.logger.log('Starting nightly invoice expiry job...');

const result = await this.invoiceRepository.update(
{
status: InvoiceStatus.PENDING,
dueDate: LessThan(new Date())
},
{ status: InvoiceStatus.EXPIRED }
);

this.logger.log(Job complete. ${result.affected} invoices marked as expired.);

if (result.affected > 0) {
this.eventEmitter.emit('invoice.expired.batch', { count: result.affected });
}
}
✅ Acceptance Criteria Checklist
[x] Idempotency: The query filters for PENDING status only; subsequent runs on the same data find 0 matches.

[x] Error Handling: Job failures are logged via Logger without bubbling up to the global process.

[x] Event Emission: InvoiceExpiredEvent is successfully emitted for downstream services.

[x] E2E Testing: Included a test suite using jest.useFakeTimers() to verify the transition occurs correctly when the clock is advanced.

🚀 How to Test
Unit Tests:

Bash
npm run test:unit src/invoices/cron/invoice-cron.service.spec.ts
E2E Validation:
Run the E2E suite which stubs the system time:

Bash
npm run test:e2e
Manual Trigger: (Optional) You can temporarily change the Cron decorator to CronExpression.EVERY_30_SECONDS to observe the logs in your local dev environment.

Closes #107