Add three indexes to prevent full table scans as session volume grows:
- impersonation_sessions(staff_id, status) for active-session lookup
- impersonation_sessions(client_id) for existing-session check
- impersonation_audit_logs(session_id) for audit log lookup by session
Migration 0011 applied and verified on dev database.
Co-Authored-By: Paperclip <noreply@paperclip.ing>