- Update BillingSummary.Breakdown field type from map[string]interface{} to map[string]any
- Update Transaction.Metadata field type from map[string]interface{} to map[string]any
- Update RatePlanCurrencyIntegrator subscription and billing metadata to use map[string]any
- Update handler request/response types to use map[string]any (CreateRatePlanRequest, UpdateRatePlanRequest,
- Move Repository, ExchangeRateProvider, BillingService, and AnalyticsService interfaces from types.go to new interface.go
- Move BillingSummary and CurrencyUsageStats structs to interface.go
- Move CurrencyModel, ExchangeRateModel, and TransactionModel to new models.go
- Move GORM hooks (BeforeCreate, BeforeUpdate) and TableName methods to models.go
- Remove redundant comments from exchange_rate_service.go