_buildMrrReferrersSubquery(postId, options) {
        const knex = this.knex;
        let subquery = knex('members_subscription_created_events as msce')
            .select('msce.referrer_source as source')
            .sum('mpse.mrr_delta as mrr')
            .join('members_paid_subscription_events as mpse', function () {
                this.on('mpse.subscription_id', '=', 'msce.subscription_id');
                // Ensure we join on member_id as well for accuracy if subscription_id isn't unique across members? (Safeguard)
                this.andOn('mpse.member_id', '=', 'msce.member_id');
            })
            .where('msce.attribution_id', postId)
            .where('msce.attribution_type', 'post')
            .groupBy('msce.referrer_source');

        // Apply date filter to the paid conversion event timestamp
        this._applyDateFilter(subquery, options, 'msce.created_at');
        return subquery;
    }
Build subquery for MRR sum per referrer for a specific post.
(MRR from paid conversions attributed to this Post/Referrer)
@private
@param {string} postId
@param {StatsServiceOptions} options
@returns {import('knex').Knex.QueryBuilder}