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.

server/api/helcim/customer.post.js

@@ -62,6 +62,7 @@ export default defineEventHandler(async (event) => {
             name: body.name,
             circle: body.circle,
             contributionAmount: body.contributionAmount,
+            billingCadence: body.contributionAmount === 0 ? 'monthly' : body.cadence,
             helcimCustomerId: customerData.id,
             helcimCustomerCode: customerData.customerCode,
             status: 'pending_payment',
@@ -76,6 +77,7 @@ export default defineEventHandler(async (event) => {
         name: body.name,
         circle: body.circle,
         contributionAmount: body.contributionAmount,
+        billingCadence: body.contributionAmount === 0 ? 'monthly' : body.cadence,
         helcimCustomerId: customerData.id,
         helcimCustomerCode: customerData.customerCode,
         status: 'pending_payment',