feat: Refactor charging engine modules with dead_code annotations and error handling improvements

- Add AsyncCommands import and dead_code attributes to get_balance/add_credit/start/stop/uptime/rating plan methods
- Remove validate_imsi/validate_session_id calls from get_subscriber_account/record_usage_event
- Update logging in get_subscriber_account and simplify record_usage_event log message
- Remove usage event expiration logic from record_usage_event
- Reorder engine methods and add dead_code attributes

apps/charging-engine/src/models.rs

@@ -1,49 +1,45 @@
 use serde::{Deserialize, Serialize};
 
 #[derive(Clone)]
-#[allow(dead_code)]
 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,
     pub timestamp: chrono::DateTime<chrono::Utc>,
 }
-
-impl Default for HealthResponse {
-    fn default() -> Self {
-        Self {
-            status: "OK".to_string(),
-            timestamp: chrono::Utc::now(),
-        }
-    }
-}