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/utils/schemas.js

@@ -63,6 +63,7 @@ export const helcimCustomerSchema = z.object({
   email: z.string().trim().toLowerCase().email(),
   circle: z.enum(['community', 'founder', 'practitioner']).optional().default('community'),
   contributionAmount: z.number().int().min(0).optional(),
+  cadence: z.enum(['monthly', 'annual']).default('monthly'),
   agreedToGuidelines: z.literal(true)
 })