#30 When unix auth is turned on, use PAM as authentication method

This feature has been implemented. PAM authentication is now used as the backend for Unix authentication. Key components implemented: - PAM authentication module (src/pam_auth.cpp) - Integration with UserManager (src/user_manager.cpp) - CMake build system integration (cmake/FindPAM.cmake) - Configuration documentation (PAM_AUTHENTICATION.md) The implementation provides secure system-level authentication without storing passwords in the application.