fix(stripe): never rewind billing period on out-of-order webhooks (D5)

Stripe doesn't guarantee event order. A late customer.subscription.
updated carrying an older period overwrote a newer current_period_end,
so graceExceeded flipped a paid member to 'late' and fired a false
'dues lapsed' email + access loss. applySubscription now reads the
stored period and keeps the later of stored vs incoming
(laterPeriodEnd, pure + tested); status still tracks latest sub.status
but grace is computed from the monotonic period.

__tests__/stripe-webhook-logic.test.ts

@@ -8,6 +8,7 @@ import {
   memberStatusPatch,
   stripeInvoiceToPaymentRow,
   minorToMajor,
+  laterPeriodEnd,
 } from '@/lib/stripe/webhook-logic'
 
 describe('isoFromUnix', () => {
@@ -123,6 +124,22 @@ describe('stripeInvoiceToPaymentRow', () => {
   })
 })
 
+describe('laterPeriodEnd (never rewind on out-of-order events)', () => {
+  const older = '2026-05-01T00:00:00.000Z'
+  const newer = '2026-06-01T00:00:00.000Z'
+  it('keeps the later of stored vs incoming regardless of arrival order', () => {
+    expect(laterPeriodEnd(newer, older)).toBe(newer) // stale event arrives late
+    expect(laterPeriodEnd(older, newer)).toBe(newer) // normal advance
+    expect(laterPeriodEnd(newer, newer)).toBe(newer)
+  })
+  it('handles nulls (first event / canceled sub with no period)', () => {
+    expect(laterPeriodEnd(null, newer)).toBe(newer)
+    expect(laterPeriodEnd(newer, null)).toBe(newer)
+    expect(laterPeriodEnd(null, null)).toBeNull()
+    expect(laterPeriodEnd(undefined, undefined)).toBeNull()
+  })
+})
+
 describe('minorToMajor', () => {
   it('divides by 100, treats nullish as 0', () => {
     expect(minorToMajor(2500)).toBe(25)