fix: auth service session memory leak

[sigfawn]

This PR fixes a memory leak in auth.service.ts where expired sessions were never removed from the activeSessions map.

1. Automatic Session Cleanup

• Problem: The activeSessions map grew indefinitely because cleanupExpiredSessions() was never called automatically.
• Fix: Added a background interval (every 5 minutes) to remove expired sessions.
• Mechanism: Uses setInterval(...).unref() to ensure the cleanup process runs in the background without blocking the Node.js process from exiting (crucial for CLI tools and tests).

Result: Stable memory usage for long-running authentication services.