fix(helcim): read dateBilling on subscription CREATE to populate next-billing cache

Helcim returns next-charge as `dateBilling` on POST /subscriptions, but the
two CREATE sites were reading `subscription.nextBillingDate`, leaving
`member.nextBillingDate` empty after every signup and free→paid upgrade.
The lazy refresh in subscription.get.js (which already accepts both shapes)
masked it on next account-page load, so renders eventually populated — but
the success response we returned to the client also had `nextBillingDate:
undefined`. Mirror the GET-side resolution at both CREATE sites: prefer
`dateBilling`, fall back to `nextBillingDate`. Existing Number.isNaN guard
unchanged; defensively rejects malformed strings from either field.

tests/server/api/update-contribution.test.js

@@ -261,6 +261,30 @@ describe('update-contribution endpoint — Case 1 (free→paid)', () => {
     expect(result.message).toBe('Successfully upgraded to paid tier')
   })
 
+  it('reads dateBilling from Helcim CREATE response and threads it to $set + response', async () => {
+    setMember(freeMember)
+    getHelcimPlanId.mockReturnValue('111')
+    getHelcimCustomer.mockResolvedValue({ customerCode: 'code-1' })
+    listHelcimCustomerCards.mockResolvedValue([{ id: 'card-1' }])
+    createHelcimSubscription.mockResolvedValue({ data: [{ id: 'sub-new', status: 'active', dateBilling: '2026-05-18' }] })
+    Member.findByIdAndUpdate.mockResolvedValue({})
+
+    const event = createMockEvent({
+      method: 'POST',
+      path: '/api/members/update-contribution',
+      body: { contributionAmount: 15, cadence: 'monthly' },
+    })
+
+    const result = await handler(event)
+
+    expect(Member.findByIdAndUpdate).toHaveBeenCalledWith(
+      'member-c1',
+      { $set: expect.objectContaining({ nextBillingDate: new Date('2026-05-18') }) },
+      { runValidators: false }
+    )
+    expect(result.subscription.nextBillingDate).toBe('2026-05-18')
+  })
+
   it('annual: calls createHelcimSubscription with annual plan id and recurringAmount 180, persists billingCadence annual', async () => {
     setMember(freeMember)
     getHelcimPlanId.mockReturnValue('222')