IA-973 : `feat: Invoice paid email on payment success, customer.created/updated handling`

MR Description

Summary

Three improvements to the Stripe webhook service: outbound invoice paid email notification after successful payment, parent-to-customer linking on customer.created / customer.updated events, and a dev-mode fallback when Stripe signature verification fails.

Changes

Invoice Paid Email Notification

• processInvoicePaymentSucceeded() now calls NotificationServiceClient.sendInvoicePaidNotification() at the end of successful processing
• Builds InvoicePaidNotificationRequestDTO with period dates, plan name, and card last 4 digits from the charge object
• Zero-amount (trial) invoices early-return to avoid duplicate/unnecessary processing
• Added NotificationServiceClient — posts to POST /api/user-management/notifications/invoice-paid via RestTemplate
• Added InvoicePaidNotificationRequestDTO record Customer Linking
• Added processCustomerCreated() and processCustomerUpdated() handlers — both read parentUuid from Stripe customer metadata and call ParentServiceClient.updateGatewayCustomerId()
• processCheckoutSessionCompleted() also links parent to Stripe customer via session metadata
• Added customer.created and customer.updated to the webhook event router switch
• Added both methods to StripeWebhookService interface
• Added ParentServiceClient with:
  • updateGatewayCustomerId(parentUuid, stripeCustomerId)
  • getParentEmailByGatewayCustomerId(gatewayCustomerId)