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.

server/api/invite/accept.post.js

@@ -59,6 +59,7 @@ export default defineEventHandler(async (event) => {
     location: body.location || undefined,
     circle: body.circle,
     contributionAmount: body.contributionAmount,
+    billingCadence: body.contributionAmount === 0 ? 'monthly' : body.cadence,
     bio: body.motivation || undefined,
     status: body.contributionAmount === 0 ? 'active' : 'pending_payment',
     helcimCustomerId: helcimCustomer?.id,