function calculateMemberTier(member, freeTier) {
    const activeSubscriptions = member.subscriptions.filter(sub => sub.status === 'active');
    if (activeSubscriptions.length === 0) {
        return freeTier;
    }
    if (activeSubscriptions.length === 1) {
        return activeSubscriptions[0].tier;
    }
    return null; // More than one active subscription
}
Calculate the member's active tier.
@param {object} member - The member object.
@param {object} freeTier - The free tier object.
@returns {string|null} - The member's active tier, or null if the member has more than one active subscription.