- Add WebhookHandler with Stripe webhook validation and event processing
- Add webhook event handlers: payment succeeded, failed, canceled, and dispute created
- Update transaction status and subscriber balance on successful payments
- Create alerts for failed payments and disputes with appropriate severity levels
- Add sendTopUpNotification to create notifications for successful top-ups
- Add PaymentController
- Add StripeGateway with secret key, webhook secret, and HTTP client with 30s timeout
- Add ProcessPayment to create payment intents with amount conversion to cents and metadata
- Add CreateCustomer to create Stripe customers with subscriber details and IMSI metadata
- Add ProcessRefund to process full or partial refunds via payment intent ID
- Add GetPaymentStatus to retrieve payment intent status and details
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