Function effective_confidence 

pub fn effective_confidence(
    base: f32,
    decay_rate: f32,
    last_validated_at: &str,
) -> f32

Calculate effective confidence with time-based decay.

effective = base_confidence - (decay_rate * days_since_validation)

Clamped to [0.0, 1.0]. If last_validated_at is invalid or missing, returns base_confidence unchanged (no decay).