- Remove ChurnRiskLevel, ChurnPrediction, ChurnMetrics, and ChurnFactor type definitions from churn_service.go
- Simplify NewChurnAnalysisService constructor to single-line initialization
- Refactor PredictChurn to use analyzeFactors, calculateScore, RiskLevelFromScore, generateReasons, and RecommendationsForRisk helper functions
- Simplify GetChurnMetrics by using periodDates helper and inline database queries
- Replace GetChurnFactors implementation with DefaultChurnFactors helper
- Fix GetRevenueAnalytics return type by dereferencing metrics struct
- Fix getCarrierMetrics to use int64 for Count and convert to int for ActiveCarriers
- Rename ExchangeRateProvider to RateProviderType for consistency
- Change RatePlanCurrencyIntegrator.exchangeService from pointer to interface type
- Add function documentation comments for NewRatePlanCurrencyIntegrator, SubscribeToPlanWith