Implement Paystack Payment Gateway Integration

## Overview

Implement Paystack payment gateway integration to enable secure and reliable online payments within Ipayecart.

The integration should support the complete payment lifecycle, from initializing a transaction to verifying payment status and handling Paystack webhook events.

## Requirements

* Integrate the Paystack API for payment initialization.
* Support customer payment through Paystack Checkout.
* Implement transaction verification after payment.
* Implement secure webhook handling for payment events.
* Validate webhook signatures to prevent unauthorized requests.
* Update order/payment status based on verified transaction results.
* Prevent duplicate payment processing through idempotency checks.
* Store relevant transaction references and payment metadata.
* Handle failed, abandoned, and successful transactions appropriately.
* Provide clear and consistent API responses for payment operations.
* Keep Paystack secret credentials in environment variables and never expose them in source control.
* Add appropriate logging and error handling without exposing sensitive payment information.

## Proposed Flow

1. Customer creates an order.
2. Ipayecart initializes a Paystack transaction.
3. Customer completes payment through Paystack.
4. Paystack sends the transaction result/webhook event.
5. Ipayecart validates the webhook signature.
6. The transaction is verified with Paystack.
7. The payment record is updated.
8. The corresponding order is marked as paid only after successful verification.

## Acceptance Criteria

* [ ] Payment transactions can be initialized successfully.
* [ ] Customers can complete payments through Paystack.
* [ ] Transactions can be verified successfully.
* [ ] Paystack webhooks are securely validated.
* [ ] Successful payments update the corresponding order correctly.
* [ ] Failed or abandoned payments are handled correctly.
* [ ] Duplicate webhook events do not result in duplicate order/payment processing.
* [ ] Sensitive Paystack credentials are not committed to the repository.
* [ ] Automated tests cover the main payment scenarios.
* [ ] API documentation is updated.

## Security Considerations

Payment status must never be trusted solely from the client-side response. The backend should verify the transaction with Paystack before marking an order as successfully paid.

Paystack secret keys and other sensitive configuration values must be managed through environment variables or a secure secrets-management solution.

Start by locating the order creation, payment, and webhook entry points in the repository, then trace how payment and order status are stored. Implement and test the full Paystack lifecycle described in the issue, including verification, signature validation, idempotency, failure handling, and environment-based secrets; done means all listed acceptance criteria pass and API documentation is updated.

Written by the indexing model from the issue text.