Create user

Creates a new user for the project. Pass type: "shared" for a shared user (server assigns a phone number from the Cosmos pool, enforces maxSharedUsers, and returns the existing record if phoneNumber or email matches an active shared user). Pass type: "dedicated" for a dedicated user, which requires assignedPhoneNumber to match an iMessage line owned by the project; the operation is idempotent on the full (phoneNumber, assignedPhoneNumber) tuple — calling it again with the same tuple returns the existing row, while calling it with the same phoneNumber but a different assignedPhoneNumber creates a new row (the same user phone may be assigned to multiple lines). firstName, lastName, and email are optional in both cases. Requires Authorization: Basic base64(projectId:projectSecret).