feat(billing): P1b pure Stripe dues logic + 10 tests

lib/stripe-logic.ts (no SDK/Supabase/React): pinned STRIPE_API_VERSION;
stripeStatusIsPaid; duesMemberStatus (active/trialing->current; past_due
-> current within grace else late; canceled/unpaid/incomplete->late;
NEVER inactive; unknown->null no-op); graceExceeded (fail-safe on
missing/invalid period); priceIdForTier; isStripeConfigured. Suite 479.

__tests__/stripe-logic.test.ts

@@ -0,0 +1,78 @@
+import { describe, it, expect } from 'vitest'
+import {
+  STRIPE_API_VERSION,
+  stripeStatusIsPaid,
+  duesMemberStatus,
+  graceExceeded,
+  priceIdForTier,
+  isStripeConfigured,
+} from '@/lib/stripe-logic'
+
+describe('STRIPE_API_VERSION', () => {
+  it('is pinned (conscious upgrades only)', () => {
+    expect(STRIPE_API_VERSION).toBe('2026-04-22.dahlia')
+  })
+})
+
+describe('stripeStatusIsPaid', () => {
+  it('active/trialing are paid; everything else is not', () => {
+    expect(stripeStatusIsPaid('active')).toBe(true)
+    expect(stripeStatusIsPaid('trialing')).toBe(true)
+    for (const s of ['past_due', 'canceled', 'unpaid', 'incomplete', 'incomplete_expired', '', null, undefined]) {
+      expect(stripeStatusIsPaid(s as string)).toBe(false)
+    }
+  })
+})
+
+describe('duesMemberStatus', () => {
+  it('paid statuses -> current', () => {
+    expect(duesMemberStatus('active', false)).toBe('current')
+    expect(duesMemberStatus('trialing', true)).toBe('current')
+  })
+  it('past_due respects the grace window', () => {
+    expect(duesMemberStatus('past_due', false)).toBe('current')
+    expect(duesMemberStatus('past_due', true)).toBe('late')
+  })
+  it('terminal/failed statuses -> late, NEVER inactive', () => {
+    for (const s of ['canceled', 'unpaid', 'incomplete', 'incomplete_expired']) {
+      expect(duesMemberStatus(s, false)).toBe('late')
+    }
+  })
+  it('unknown/empty -> null (no change, never blind overwrite)', () => {
+    expect(duesMemberStatus(undefined, true)).toBeNull()
+    expect(duesMemberStatus('something_new', true)).toBeNull()
+  })
+})
+
+describe('graceExceeded', () => {
+  const end = '2026-05-01T00:00:00Z'
+  it('false within the window, true past it', () => {
+    expect(graceExceeded(end, 7, '2026-05-05T00:00:00Z')).toBe(false)
+    expect(graceExceeded(end, 7, '2026-05-08T00:01:00Z')).toBe(true)
+    expect(graceExceeded(end, 0, '2026-05-01T00:00:01Z')).toBe(true)
+  })
+  it('missing/invalid period end is never "exceeded" (fail safe)', () => {
+    expect(graceExceeded(null, 7, '2030-01-01T00:00:00Z')).toBe(false)
+    expect(graceExceeded('nope', 7, '2030-01-01T00:00:00Z')).toBe(false)
+  })
+})
+
+describe('priceIdForTier', () => {
+  const cfg = { prices: { plus: 'price_1', basic: ' ', associate: undefined } }
+  it('returns the mapped non-empty price or null', () => {
+    expect(priceIdForTier(cfg, 'plus')).toBe('price_1')
+    expect(priceIdForTier(cfg, 'basic')).toBeNull()
+    expect(priceIdForTier(cfg, 'associate')).toBeNull()
+    expect(priceIdForTier(cfg, 'missing')).toBeNull()
+    expect(priceIdForTier(null, 'plus')).toBeNull()
+  })
+})
+
+describe('isStripeConfigured', () => {
+  it('needs publishable key + secret ref + >=1 mapped price', () => {
+    expect(isStripeConfigured(null)).toBe(false)
+    expect(isStripeConfigured({ publishable_key: 'pk', secret_key_ref: 's', prices: {} })).toBe(false)
+    expect(isStripeConfigured({ publishable_key: 'pk', prices: { plus: 'price_1' } })).toBe(false)
+    expect(isStripeConfigured({ publishable_key: 'pk', secret_key_ref: 's', prices: { plus: 'price_1' } })).toBe(true)
+  })
+})