- Add auth_test.go with setupTestDB/setupAuthService helpers using SQLite in-memory database and GORM AutoMigrate
- Add TestAuthService_Register with duplicate username/email validation and invalid role handling
- Add TestAuthService_Login with invalid username/password checks and inactive user validation
- Add TestJWTService_GenerateToken with database session verification and token storage validation
- Add TestJWTService_ValidateToken with invalid
- Add ChargingService with database and charging engine client dependencies
- Add GetSystemStats to retrieve active sessions, accounts, blocked users, and uptime metrics
- Add GetHealthStatus to check Redis connection, sync status, and memory usage
- Add GetUsageStats to get subscriber usage statistics with data/voice/SMS volumes, costs, and trends
- Add GetRealTimeUsage to track current session and today's usage metrics