Atomically find or create a customer and upsert an external membership in one call. Matches on first_name + last_name + birthdate (+ email if provided). 0 matches → creates customer + membership (201). 1 match → upserts membership for existing customer (200 or 201). 2+ matches → 422 with list of matching customer IDs for manual resolution. If external_id is already assigned to a different customer → 422 with existing_customer_id. Status is automatically derived from expires_at: null/future → active, past → expired. Every save is recorded in the audit log (objectType = SIT_ExternalMembership).