fix(validator): restrict entry_sorting_order to database enum values

ValidateUserModification reused ValidateEntryOrder, which accepts nine
sorting fields valid for the entry-listing order query parameter, while
the users.entry_order column is an entry_sorting_order enum allowing
only published_at and created_at. Requests such as
PUT /v1/users/{id} with entry_sorting_order=title passed validation and
failed in PostgreSQL, returning 500 instead of 400.

Validate the user preference against the enum values with a dedicated
validateEntrySortingOrder function and keep ValidateEntryOrder for the
entry-listing endpoint.