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/helcim-subscription.test.js

@@ -166,6 +166,41 @@ describe('helcim subscription endpoint', () => {
     expect(Member.findById).toHaveBeenCalledWith('member-2')
   })
 
+  it('reads dateBilling from Helcim CREATE response and threads it to $set + response', async () => {
+    requireAuth.mockResolvedValue(undefined)
+    requiresPayment.mockReturnValue(true)
+    getHelcimPlanId.mockReturnValue('99999')
+
+    const mockMember = {
+      _id: 'member-db',
+      email: 'datebilling@example.com',
+      name: 'DateBilling User',
+      circle: 'founder',
+      contributionAmount: 15,
+      status: 'active',
+    }
+    Member.findOneAndUpdate.mockResolvedValue({ _id: 'member-db', status: 'pending_payment' })
+    Member.findById.mockResolvedValue(mockMember)
+    createHelcimSubscription.mockResolvedValue({
+      data: [{ id: 'sub-x', status: 'active', dateBilling: '2026-06-01' }]
+    })
+
+    const event = createMockEvent({
+      method: 'POST',
+      path: '/api/helcim/subscription',
+      body: { customerId: 'cust-1', contributionAmount: 15, customerCode: 'code-1', cardToken: 'tok-123', cadence: 'monthly' }
+    })
+
+    const result = await subscriptionHandler(event)
+
+    expect(Member.findOneAndUpdate).toHaveBeenCalledWith(
+      { helcimCustomerId: 'cust-1' },
+      { $set: expect.objectContaining({ nextBillingDate: new Date('2026-06-01') }) },
+      { new: false, runValidators: false, projection: { status: 1 } }
+    )
+    expect(result.subscription.nextBillingDate).toBe('2026-06-01')
+  })
+
   it('annual $15 tier creates subscription with correct paymentPlanId and recurringAmount', async () => {
     requireAuth.mockResolvedValue(undefined)
     requiresPayment.mockReturnValue(true)