feat: Remove api module and relocate handlers, clean up unused code and dead_code attributes

- Delete api.rs module containing 20+ handler functions (credit management, subscriber accounts, rating plans, user blocking, system stats, metrics, engine control, error stats, sync control, health checks)
- Remove dead_code attributes from UsageEvent and UsageType in charging/types
- Remove validate_imsi helper function and dead_code attribute from errors/helpers
- Remove ConfigurationError logging case

apps/charging-engine/src/monitoring/types.rs

@@ -16,7 +16,6 @@ pub struct HealthStatus {
 }
 
 #[derive(Debug, Clone)]
-#[allow(dead_code)]
 pub struct PerformanceMetrics {
     pub connected_clients: u64,
     pub used_memory: u64,
@@ -26,7 +25,6 @@ pub struct PerformanceMetrics {
 }
 
 #[derive(Debug, Clone)]
-#[allow(dead_code)]
 pub struct ErrorStats {
     pub total_errors: u64,
     pub error_types: std::collections::HashMap<String, u64>,