feat(api): default helcim customer circle to community when omitted

Lets the join form omit circle from the signup payload (circle picker
moves to a post-signup prompt per docs/specs/join-form-redesign.md).
Mongoose Member.circle is required with no default, so the Zod
default fills it in before reaching the model.

server/utils/schemas.js

@@ -61,7 +61,7 @@ export const helcimCreatePlanSchema = z.object({
 export const helcimCustomerSchema = z.object({
   name: z.string().min(1).max(200),
   email: z.string().trim().toLowerCase().email(),
-  circle: z.enum(['community', 'founder', 'practitioner']).optional(),
+  circle: z.enum(['community', 'founder', 'practitioner']).optional().default('community'),
   contributionAmount: z.number().int().min(0).optional(),
   agreedToGuidelines: z.literal(true)
 })