Auto create a data folder for new users

Summary

Automatically create a folder under DATA_ROOT for a newly created user and set it as their scan directory. Split out of #492.

Current behaviour

A new user has no scan directory until an admin picks one. On a single-server install where each user should simply get their own library, the admin has to create a folder on the host and then assign it in the UI for every account.

Proposed change

• Admin-level setting, off by default, for example AUTO_CREATE_USER_DIRECTORY.
• When on, creating a user (admin UI, signup, or SSO provisioning) creates DATA_ROOT/<username> if it does not exist and stores it as the scan directory. Validation goes through normalize_scan_directory() from #2025.
• Do not overwrite a directory that was explicitly given on create.
• Skip silently with a logged warning if DATA_ROOT is not writable, so a read-only library mount does not break user creation.

Related: creating a folder from the directory picker, tracked separately.