Create User

Insert a new user row in the users table. • SECURITY: user_id is generated on backend, NOT from frontend input. • Uses the schema: id, provider, name, email, join_date, characters[], shows[], subscription, bypass_subscription. • If referrer_user_id is provided (e.g. from /signup?ref=USER_ID), creates a pending referral row; does not block signup on failure. • Returns a JSON payload on success with backend-generated user_id.