feat: Add data models for charging engine API endpoints

- Add AppState struct to hold shared ChargingEngine instance
- Add CreditCheckRequest and CreditCheckResponse for credit validation
- Add DeductRequest for byte usage deduction
- Add AddCreditRequest for credit top-up operations
- Add BalanceResponse for IP address balance queries
- Add HealthResponse with default implementation for health check endpoint
- Derive Serialize/Deserialize traits for API request/response types

apps/charging-engine/src/models.rs

@@ -0,0 +1,48 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone)]
+pub struct AppState {
+    pub charging_engine: std::sync::Arc<crate::charging::ChargingEngine>,
+}
+
+#[derive(Deserialize)]
+pub struct CreditCheckRequest {
+    pub bytes_requested: u64,
+}
+
+#[derive(Serialize)]
+pub struct CreditCheckResponse {
+    pub allowed: bool,
+    pub remaining_bytes: i64,
+}
+
+#[derive(Deserialize)]
+pub struct DeductRequest {
+    pub bytes_used: u64,
+}
+
+#[derive(Deserialize)]
+pub struct AddCreditRequest {
+    pub bytes_to_add: u64,
+}
+
+#[derive(Serialize)]
+pub struct BalanceResponse {
+    pub ip: String,
+    pub balance_bytes: i64,
+}
+
+#[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(),
+        }
+    }
+}