* server-side comments view tracking, model structure, mutation
* full commentsViewedAt refactor, adjust comment creation and injection, adjust item navigation
* update server-side tracking only if there's a change, light cleanup
* coalesce meCommentsViewedAt to the item's createdAt, wip PoC comment outlining
* don't update cache on item visit, use useRoot hook for outlining
* add meCommentsViewedAt to root, better naming, light cleanup
* better timestamp logic and comparisons, add lastCommentAt to root item object, added TODOs
* fix: track commentsViewedAt only for root item, use topLevelId to fetch live comments only for the current item
* only track commentsViewedAt for root item, light cleanup
* light cleanup, correct live comments timestamp deps
* worker: on midnight, untrack items that were never viewed and had no comments in the last 21 days
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