refactor(stripe): extract + unit-test webhook decision logic (R2)

The webhook's money/status mapping (Basil field relocations, period
extraction, customer-id unwrap, status patch, invoice->payment row,
minor->major) lived inline in the route with zero test coverage - the
biggest untested critical path in the codebase. Extracted to a pure,
SDK-free lib/stripe/webhook-logic.ts (structural input types) and
refactored the route to only do signature verify + network + DB writes.
12 new tests cover the Basil edges and link/unlink + defaults. Behavior
unchanged; suite 514.

lib/stripe/webhook-logic.ts

@@ -0,0 +1,101 @@
+// Pure decision + mapping logic for the Stripe webhook. No Stripe SDK, no
+// Supabase, no I/O — structural input types only, so every branch is unit-
+// testable in isolation. The route handler keeps only signature verify,
+// network calls, and DB writes; all "what should we derive/record" logic
+// lives here. Status mapping itself stays in lib/stripe-logic.ts
+// (duesMemberStatus / graceExceeded); this module is the webhook-shaped glue
+// around it, including the Stripe Basil (2025-03-31 / 2026-04-22.dahlia)
+// field relocations that are easy to get wrong and must be tested.
+
+type CustomerRef = string | { id?: string | null } | null | undefined
+
+type SubscriptionLike = {
+  items?: { data?: Array<{ current_period_end?: number } | null | undefined> }
+}
+
+type InvoiceLike = {
+  id?: string | null
+  amount_paid?: number | null
+  amount_due?: number | null
+  currency?: string | null
+  description?: string | null
+  customer_email?: string | null
+  parent?: { subscription_details?: { metadata?: Record<string, string> | null } | null } | null
+  lines?: { data?: Array<{ period?: { end?: number } | null } | null> } | null
+}
+
+export function isoFromUnix(s: number | null | undefined): string | null {
+  return typeof s === 'number' && s > 0 ? new Date(s * 1000).toISOString() : null
+}
+
+// Stripe customer is either an id string or an expanded object.
+export function customerIdOf(customer: CustomerRef): string | null {
+  if (typeof customer === 'string') return customer || null
+  return customer?.id ?? null
+}
+
+// Basil moved the billing period off the top-level Subscription onto each
+// item. Dues are a single-price subscription, so item 0 carries it.
+export function subscriptionPeriodEnd(sub: SubscriptionLike | null | undefined): string | null {
+  return isoFromUnix(sub?.items?.data?.[0]?.current_period_end)
+}
+
+// Basil moved invoice subscription metadata under
+// invoice.parent.subscription_details.metadata.
+export function invoiceMetadataMemberId(inv: InvoiceLike | null | undefined): string | undefined {
+  return inv?.parent?.subscription_details?.metadata?.member_id || undefined
+}
+
+// Next renewal date for the renewal receipt: the paid invoice line's period
+// end. Absent on some invoices -> null (email simply omits the date).
+export function invoiceLinePeriodEnd(inv: InvoiceLike | null | undefined): string | null {
+  return isoFromUnix(inv?.lines?.data?.[0]?.period?.end)
+}
+
+// Build the space_members patch for a billing-driven status change. Only
+// current<->late ever moves (the route additionally constrains the UPDATE to
+// rows already in those states, so inactive/unverified are never touched).
+// Returns null when there is no status to apply.
+export function memberStatusPatch(
+  desired: 'current' | 'late' | null | undefined,
+  nowIso: string,
+): { status: 'current' | 'late'; last_paid_at?: string } | null {
+  if (!desired) return null
+  if (desired === 'current') return { status: 'current', last_paid_at: nowIso }
+  return { status: 'late' }
+}
+
+// Stripe amounts are in minor units; the payments ledger stores major units
+// (numeric, matching the existing PayPal/CSV rows). Centralized so the cents
+// division + currency casing + link status can be tested once.
+export function stripeInvoiceToPaymentRow(args: {
+  inv: InvoiceLike
+  spaceId: string
+  memberId: string | null
+  eventId: string
+  nowIso: string
+}) {
+  const { inv, spaceId, memberId, eventId, nowIso } = args
+  const linked = memberId ? 'linked' : 'unlinked'
+  return {
+    space_id: spaceId,
+    member_id: memberId,
+    platform: 'stripe' as const,
+    amount: (inv.amount_paid ?? 0) / 100,
+    currency: (inv.currency ?? 'usd').toUpperCase(),
+    description: inv.description ?? 'Stripe membership dues',
+    status: linked,
+    link_status: linked,
+    external_id: inv.id ?? null,
+    payer_email: inv.customer_email ?? null,
+    from_identifier: inv.customer_email ?? null,
+    transaction_date: nowIso,
+    raw_data: { event_id: eventId, invoice: inv.id ?? null },
+  }
+}
+
+// Major-units amount for the dues email (amount_paid for renewals,
+// amount_due for failed-payment notices).
+export function minorToMajor(amount: number | null | undefined): number {
+  return (amount ?? 0) / 100
+}