Part of kcenon/common_system#456

What

Change the CMake export namespace from DatabaseSystem:: to database_system::.

• Current: NAMESPACE DatabaseSystem::
• Expected: NAMESPACE database_system::
• Scope: CMakeLists.txt install(EXPORT) rules, Config template

Why

Ecosystem convention is <name>:: with snake_case. Standardizing matches vcpkg port naming and find_package() package name.

How

1. Change NAMESPACE DatabaseSystem:: to NAMESPACE database_system:: in install(EXPORT)
2. Add backward compatibility aliases in Config template
3. Update portfile if needed

Acceptance Criteria

• Primary namespace is database_system::
• Backward compatibility aliases provided for DatabaseSystem::
• Export file name remains database_system-targets.cmake (already correct)