- 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
