Introduced a new column `default_new_users` in the library table to
facilitate automatic assignment of default libraries to new regular users.
When a new user is created, they will now be assigned to libraries marked
as default, enhancing user experience by ensuring they have immediate access
to essential resources. Additionally, updated the user repository logic
to handle this new functionality and modified the user creation validation
to reflect that library selection is optional for non-admin users.
Signed-off-by: Deluan <deluan@navidrome.org>
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