I would make an additional Tag model so the entities would be Tag, User, List, ListItem, with one pivot list_item_user_tags.

I would leave the users table as it is, then the tags table - id, name, user_id (so the tags are user-specific); lists table - id, name; list_items table - id, name; list_item_user_tags table - id, list_id, list_item_id, tag_id, user_id.