feat(auth): Add LDAP authentication support

[adamoutler]

This PR introduces a comprehensive LDAP and Active Directory authentication provider for NetAlertX.

Key features include:

• Native LDAP/AD login support configured directly from the UI or via Environment Variables for zero-touch deployment.
• Full StartTLS and LDAPS support with strict certificate verification options.
• Dynamic fallback to local admin if LDAP is unreachable or disabled.
• Fully automated comprehensive Docker UI, backend, and API testing for LDAP workflows.
• Dedicated documentation detailing Active Directory / OpenLDAP filtering and best practices.

(Authored in collaboration with AI)

Summary by CodeRabbit

• New Features

  • LDAP/Active Directory authentication with UI login support, plugin test command, and in‑app idle notifications.

• Security Enhancements

  • CSRF protection for login, stronger session cookie validation (HMAC), and server‑side login rate limiting.

• Documentation

  • New comprehensive LDAP configuration guide with examples and best practices.

• Tests

  • Extensive unit, integration, Docker and UI tests for LDAP/auth flows; CI test toggle for LDAP and updated test runtime setup (adds LDAP client dependency).