Eliminate all code, tests, and utilities related to the legacy 'unclaimed' user
scope. All API endpoints, document and audiobook storage, and access logic now
require a valid authenticated userId and only operate on resources owned by that
user. Remove related helper functions, test cases, and conditional flows for
anonymous/unclaimed data. Update types, client APIs, and UI logic to reflect
that only 'user' scope is supported. This simplifies ownership checks and
removes ambiguity around document and audiobook access.
