#438 Implement Enterprise SSO (SAML/OIDC) for platform authentication

Add enterprise SSO support for Highway platform. Requirements: (1) SAML 2.0 support for enterprise IdPs (Okta, Azure AD, etc.), (2) Generic OIDC provider support, (3) Create api/oauth_handlers/oauth2_platform.py with platform_auth_bp blueprint, (4) Support tenant-specific SSO configuration, (5) Integrate with existing RBAC system. Reference: Issue #437 tracked incomplete placeholder code that was removed.