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/main.rs

@@ -1,9 +1,9 @@
-mod api;
 mod charging;
 mod monitoring;
 mod errors;
 mod models;
 mod routes;
+mod handlers;
 
 use anyhow::Result;
 use std::sync::Arc;