Reverse daily-code lookup (Issue #724 / Modo Fácil 1.2): resolve the child
whose ACTIVE check-in computes to the guardian's typed daily code. The code
is validated against the server-held secret; a matching student is returned
with the checkInId needed to perform check-out. Invalid codes produce a 404
(audited server-side) so the receptionist can retry without lockout.
Reverse daily-code lookup (Issue #724 / Modo Fácil 1.2): resolve the child whose ACTIVE check-in computes to the guardian's typed daily code. The code is validated against the server-held secret; a matching student is returned with the checkInId needed to perform check-out. Invalid codes produce a 404 (audited server-side) so the receptionist can retry without lockout.