#59 API: List Active Sessions

Implement endpoint to list all active sessions for a user or tenant. Endpoint: GET /api/v1/sessions Query Parameters: - user_id: Filter by user (optional, admin only) - active_only: Only show active sessions (default: true) - page: Page number (default: 1) - per_page: Results per page (default: 50) Response: List of sessions with metadata Required Permissions: tenant_admin for all sessions, users can see own sessions Implementation Requirements: - Show session ID, user, IP address, user agent - Show last activity timestamp - Show created timestamp - Show expiration time - Highlight current session - Group by user for admin view With ultra attention to details and correctness. Before closing: write comment summarizing implementation with screenshot.