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 and enforces maxSharedUsers). A shared user is identified by phoneNumber within the project: re-creating with an existing active phoneNumber returns that same user and updates its firstName, lastName, and email from any values supplied (omitted/null fields are left unchanged; maxSharedUsers is not re-checked on an update). 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).