PERF: Avoid full posts table scans during anonymisation (#21081)

2e78045a fixed the anonymization job so that it correctly updated self-mentions, which are not logged in the post_actions table. The solution was to scan the entire `posts` table with an `raw ILIKE` query. On sites with many posts, this can take a very long time.

This commit updates the job to take a two-pass approach:

First, we update posts based on the post_actions table. This is much more efficient than a full table scan, and takes care of all 'non-self' mentions.

Then, we make a second pass using the `raw ILIKE` approach. Since we already took care of most posts, we can scope this down to self-mentions only. By filtering the query to a specific posts.user_id, it is significantly more performant than a full table scan.

app/jobs/regular/update_username.rb

@@ -46,9 +46,21 @@ module Jobs
     def update_posts
       updated_post_ids = Set.new
 
+      # Other people mentioning this user
+      Post
+        .with_deleted
+        .joins(mentioned("posts.id"))
+        .where("a.user_id = :user_id", user_id: @user_id)
+        .find_each do |post|
+          update_post(post)
+          updated_post_ids << post.id
+        end
+
+      # User mentioning self (not included in post_actions table)
       Post
         .with_deleted
         .where("raw ILIKE ?", "%@#{@old_username}%")
+        .where("posts.user_id = :user_id", user_id: @user_id)
         .find_each do |post|
           update_post(post)
           updated_post_ids << post.id