IAV-995 : Implement payment statistics method

• Convert all Stripe payment DTOs to Java records for better immutability

• Remove 'Dto' suffix from class names:

  • PaymentDTO Payment
  • PaymentStatisticsDTO PaymentStatistics
  • SuccessfulPaymentsStatisticsDTO SuccessfulPaymentsStatistics
  • FailedPaymentsStatisticsDTO FailedPaymentsStatistics
  • RevenueStatisticsDTO RevenueStatistics

• Update all related components:

  • StripePaymentService interface
  • StripePaymentServiceImpl implementation
  • StripePaymentController endpoints
  • StripePaymentMapper

• Benefits:

  • Immutability by default
  • Reduced Lombok boilerplate
  • Automatic equals/hashCode/toString
  • Cleaner, more maintainable code
  • Full backward API compatibility

Closes IAV-995