Proper support for meta-listing pages: - Added support for URL endpoints. - Moved all category-related stuff to generic meta handling. - Generic meta-listing pages in the front end. Miscellaneous fixes: - Remove previous "ready" items before caching new ones. - Only un-"ready" pages that have includes and queries. - Better error handling in `DatabasePage` auto-update. - Make sure pages have extended data cached before running queries.

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