Function set_widget_filter 

pub async fn set_widget_filter(
    user_id: i64,
    widget_key: &str,
    filter_key: &str,
    value: &str,
) -> Result<(), Error>

Persist one filter value at preferences.dashboard.widget_filters.<widget_key>.<filter_key>.

Same read-modify-write merge as set_widget_period. A filter the user picks is sticky across reloads, tabs and devices — the dashboard is a tool people re-open, and re-picking “status = paid” every morning is the kind of papercut that makes a control panel feel disposable.