refactor: Replace fallback defaults with required environment variable validation in charging-engine auth

- Change AuthConfig::from_env() return type from Self to Result<Self, String>
- Remove default JWT_SECRET fallback, require environment variable to be set
- Add validation requiring JWT_SECRET to be at least 32 characters for security
- Replace expect() with ok_or_else in create_token timestamp calculation
- Replace expect() with map_err in DATABASE_URL environment variable loading

apps/charging-engine/src/auth.rs

@@ -21,8 +21,13 @@ pub struct AuthConfig {
 }
 
 impl AuthConfig {
-    pub fn from_env() -> Self {
-        let secret = env::var("JWT_SECRET").unwrap_or_else(|_| "default_secret_key_change_in_production".to_string());
+    pub fn from_env() -> Result<Self, String> {
+        let secret = env::var("JWT_SECRET")
+            .map_err(|_| "JWT_SECRET environment variable is required".to_string())?;
+        
+        if secret.len() < 32 {
+            return Err("JWT_SECRET must be at least 32 characters for security".to_string());
+        }
         
         // Parse API keys from comma-separated environment variable
         let api_keys_env = env::var("API_KEYS").unwrap_or_else(|_| String::new());
@@ -32,7 +37,7 @@ impl AuthConfig {
             .filter(|s| !s.is_empty())
             .collect();
         
-        Self { secret, api_keys }
+        Ok(Self { secret, api_keys })
     }
     
     pub fn validate_api_key(&self, api_key: &str) -> bool {
@@ -43,7 +48,7 @@ impl AuthConfig {
 pub fn create_token(user_id: &str, config: &AuthConfig) -> Result<String, StatusCode> {
     let expiration = chrono::Utc::now()
         .checked_add_signed(chrono::Duration::hours(24))
-        .expect("valid timestamp")
+        .ok_or_else(|| StatusCode::INTERNAL_SERVER_ERROR)?
         .timestamp() as usize;
 
     let claims = Claims {

apps/charging-engine/src/main.rs

@@ -29,7 +29,7 @@ async fn main() -> Result<()> {
     // Initialize charging engine
     let redis_url = std::env::var("REDIS_URI").unwrap_or_else(|_| "redis://127.0.0.1/".to_string());
     let database_url = std::env::var("DATABASE_URL")
-        .expect("DATABASE_URL is required (for rating plans)");
+        .map_err(|_| anyhow::anyhow!("DATABASE_URL environment variable is required (for rating plans)"))?;
     let sync_interval = std::env::var("SYNC_INTERVAL")
         .unwrap_or_else(|_| "1".to_string())
         .parse::<u64>()
@@ -47,7 +47,8 @@ async fn main() -> Result<()> {
     info!("Connected to Redis successfully");
 
     // Create application state
-    let auth_config = AuthConfig::from_env();
+    let auth_config = AuthConfig::from_env()
+        .map_err(|e| anyhow::anyhow!("Failed to initialize auth config: {}", e))?;
     let state = crate::models::AppState {
         charging_engine,
         auth_config,