DEV: Add has_many category_tag_stats in tag.rb (PR #9558)

We have the other side of this relationship already set up.

https://github.com/discourse/discourse/blob/fb20b570577edb2d9a27079a0eb4fbb2fd1cc82b/app/models/category_tag_stat.rb#L5

This addition makes this work, and it currently does not work.

Tag.joins(:category_tag_stats).where("category_tag_stats.category_id = ?", category_id)

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