calibre-server will only ever write to the database when performing an automatic
schema upgrade after a calibre update changes the layout of the database... which happens only rarely.
If you really want to prevent the database from being modified, you could always use the
environment variable "CALIBRE_OVERRIDE_DATABASE_PATH" pointed at a temporary copy which can be trashed.
Don't modify any metadata while you do this
calibre might lose track of your books!