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/models.rs

@@ -5,39 +5,33 @@ pub struct AppState {
     pub charging_engine: std::sync::Arc<crate::charging::ChargingEngine>,
 }
 
-#[allow(dead_code)]
 #[derive(Deserialize)]
 pub struct CreditCheckRequest {
     pub bytes_requested: u64,
 }
 
-#[allow(dead_code)]
 #[derive(Serialize)]
 pub struct CreditCheckResponse {
     pub allowed: bool,
     pub remaining_bytes: i64,
 }
 
-#[allow(dead_code)]
 #[derive(Deserialize)]
 pub struct DeductRequest {
     pub bytes_used: u64,
 }
 
-#[allow(dead_code)]
 #[derive(Deserialize)]
 pub struct AddCreditRequest {
     pub bytes_to_add: u64,
 }
 
-#[allow(dead_code)]
 #[derive(Serialize)]
 pub struct BalanceResponse {
     pub ip: String,
     pub balance_bytes: i64,
 }
 
-#[allow(dead_code)]
 #[derive(Serialize)]
 pub struct HealthResponse {
     pub status: String,