fix(invite): persist billingCadence at invite-accept time

Annual-choosing invitees who abandoned between accept and payment were left at billingCadence:'monthly' (the model default) while contributionAmount held an annual-unit value, rendering $180/mo in admin views. Persist the chosen cadence at Member.create time.

accept-invite.vue now sends cadence in the accept POST body; inviteAcceptSchema accepts cadence (defaults 'monthly'); accept.post.js sets billingCadence on create, forced to 'monthly' for $0 members since a free member has no billing relationship.

tests/server/api/activation-auto-flag.test.js

@@ -209,3 +209,62 @@ describe('POST /api/members/create — auto-flag wiring (3.8)', () => {
     )
   })
 })
+
+// ---------------------------------------------------------------------------
+// billingCadence persistence on /api/invite/accept
+//
+// Regression: an annual-choosing invitee who abandoned before payment was left
+// with billingCadence defaulting to 'monthly' while contributionAmount held an
+// annual-unit value (e.g. 180), rendering "$180/mo" in admin views. The handler
+// must persist the chosen cadence at Member.create time.
+// ---------------------------------------------------------------------------
+describe('POST /api/invite/accept — billingCadence persistence', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    PreRegistration.findById.mockResolvedValue({
+      _id: 'prereg-1',
+      email: 'invitee@example.com',
+      status: 'pending'
+    })
+    PreRegistration.findByIdAndUpdate.mockResolvedValue(undefined)
+    Member.findOne.mockResolvedValue(null)
+    Member.create.mockImplementation(async (data) => ({ _id: 'new-member-via-invite', ...data }))
+  })
+
+  it('persists billingCadence "annual" for an annual paid invite', async () => {
+    globalThis.validateBody.mockResolvedValue({
+      token: 'tok',
+      preRegistrationId: 'prereg-1',
+      name: 'Annual Invitee',
+      circle: 'community',
+      contributionAmount: 180,
+      cadence: 'annual'
+    })
+    createHelcimCustomer.mockResolvedValue({ id: 'cust-annual', customerCode: 'code-annual' })
+
+    const event = createMockEvent({ method: 'POST', path: '/api/invite/accept' })
+    await inviteAcceptHandler(event)
+
+    expect(Member.create).toHaveBeenCalledWith(
+      expect.objectContaining({ contributionAmount: 180, billingCadence: 'annual' })
+    )
+  })
+
+  it('forces billingCadence "monthly" for a $0 invite even when cadence is annual', async () => {
+    globalThis.validateBody.mockResolvedValue({
+      token: 'tok',
+      preRegistrationId: 'prereg-1',
+      name: 'Free Invitee',
+      circle: 'community',
+      contributionAmount: 0,
+      cadence: 'annual'
+    })
+
+    const event = createMockEvent({ method: 'POST', path: '/api/invite/accept' })
+    await inviteAcceptHandler(event)
+
+    expect(Member.create).toHaveBeenCalledWith(
+      expect.objectContaining({ contributionAmount: 0, billingCadence: 'monthly' })
+    )
+  })
+})