#642 Internal Platform Tools

Implement internal.platform.* tools for platform-level operations. Tools: - internal.platform.check_initialized - Check if platform exists - internal.platform.create_system_tenant - Create a system tenant - internal.platform.create_owner - Create platform owner - internal.platform.register_apps - Register pre-installed apps to marketplace - internal.platform.setup_secrets - Create platform secrets (SMTP, etc.) - internal.platform.mark_initialized - Mark platform as initialized Parent: #639