fix(join): persist billingCadence on /join signup

Mirror of the invite-accept fix (c3b1c59) for the self-serve /join path. A joiner who picked annual but abandoned before /api/helcim/subscription ran was left at billingCadence:'monthly' (the model default) with a cadence-unit contributionAmount, rendering $180/mo in admin views.

join.vue now sends cadence to /api/helcim/customer; helcimCustomerSchema accepts cadence (defaults 'monthly'); customer.post.js persists billingCadence in both the new-member create branch and the guest-upgrade $set branch, forced to 'monthly' for $0 signups.

tests/server/api/helcim-customer.test.js

@@ -385,4 +385,67 @@ describe('POST /api/helcim/customer', () => {
       expect(setAuthCookie).not.toHaveBeenCalled()
     })
   })
+
+  // Regression: a joiner who picks annual but abandons before
+  // /api/helcim/subscription runs must already have billingCadence persisted,
+  // otherwise contributionAmount (cadence-unit, e.g. 180) renders as "$180/mo".
+  describe('billingCadence persistence', () => {
+    it('persists billingCadence "annual" when a new member signs up annual', async () => {
+      Member.findOne.mockResolvedValue(null)
+      const event = build({
+        body: {
+          name: 'Annual Joiner',
+          email: 'annual@example.com',
+          circle: 'community',
+          contributionAmount: 180,
+          cadence: 'annual',
+          agreedToGuidelines: true
+        }
+      })
+      await customerHandler(event)
+      expect(Member.create).toHaveBeenCalledWith(
+        expect.objectContaining({ contributionAmount: 180, billingCadence: 'annual' })
+      )
+    })
+
+    it('persists billingCadence "annual" when upgrading a guest who signs up annual', async () => {
+      Member.findOne.mockResolvedValue({
+        _id: 'guest-annual',
+        email: 'guest@example.com',
+        status: 'guest'
+      })
+      const event = build({
+        body: {
+          name: 'Guest Annual',
+          email: 'guest@example.com',
+          circle: 'founder',
+          contributionAmount: 180,
+          cadence: 'annual',
+          agreedToGuidelines: true
+        }
+      })
+      await customerHandler(event)
+      expect(Member.create).not.toHaveBeenCalled()
+      const [, updatePayload] = Member.findByIdAndUpdate.mock.calls[0]
+      expect(updatePayload.$set).toMatchObject({ contributionAmount: 180, billingCadence: 'annual' })
+    })
+
+    it('forces billingCadence "monthly" for a $0 signup even when cadence is annual', async () => {
+      Member.findOne.mockResolvedValue(null)
+      const event = build({
+        body: {
+          name: 'Free Joiner',
+          email: 'free@example.com',
+          circle: 'community',
+          contributionAmount: 0,
+          cadence: 'annual',
+          agreedToGuidelines: true
+        }
+      })
+      await customerHandler(event)
+      expect(Member.create).toHaveBeenCalledWith(
+        expect.objectContaining({ contributionAmount: 0, billingCadence: 'monthly' })
+      )
+    })
+  })
 })