Creates a water intake for the specified APIUser without image-based prediction or dish selection.

The created resource is a normal image-less manual intake whose only dish is the configured water class. It is
returned in intake history with is_water: true and contributes to total_water.manual_water. The classification
is content-based, so an equivalent water-only intake created through /intake/manualInput/USERID behaves the
same way.

The request body is optional. quantity defaults to 250 ml and timestamp defaults to the user's current
localized time. A supplied timestamp is interpreted in the target user's local time.

An APIUser can create water only for themselves. An APIUserManager can create it for an APIUser they are
authorized to manage.