Billing Model

{
  userId?: ObjectId                  // Reference to User (optional)
  email: string                      // For pre-purchase lookup
  stripeCustomerId: string           // Stripe customer ID
  currentPlan: 'basic' | 'pro' | null
  oneTimePurchases: [{
    chargeId: string
    productType: 'basic' | 'pro'
    amount: number                   // in cents
    currency: string
    purchasedAt: Date
  }]
  subscriptions: [{
    stripeSubscriptionId: string
    stripePriceId: string
    status: SubscriptionStatus
    currentPeriodStart: Date
    currentPeriodEnd: Date
    cancelAtPeriodEnd: boolean
    canceledAt?: Date
  }]
  createdAt: Date
  updatedAt: Date
}

enum SubscriptionStatus {
  ACTIVE = 'active',
  PAST_DUE = 'past_due',
  UNPAID = 'unpaid',
  CANCELED = 'canceled',
  INCOMPLETE = 'incomplete',
  INCOMPLETE_EXPIRED = 'incomplete_expired',
  TRIALING = 'trialing',
  PAUSED = 'paused',
}

const billing = new Billing({
  userId: null,  // Not signed up yet
  email: session.customer_email,
  stripeCustomerId: session.customer,
  currentPlan: 'pro',
  oneTimePurchases: [{
    chargeId: charge.id,
    productType: 'pro',
    amount: charge.amount,
    currency: charge.currency,
    purchasedAt: new Date()
  }]
})
await billing.save()

const billing = await Billing.findOne({ email })
billing.userId = user._id
await billing.save()

const billing = await Billing.findOne({ userId: user._id })
const plan = billing?.currentPlan  // 'basic', 'pro', or null