Add comment_threads reverse lookup index

[raymondjacobson]

Summary

• Add a concurrent index on comment_threads (comment_id).
• Speeds comment-create trigger paths that check whether a newly inserted comment has a thread row by comment_id.

Context

The slow ETL sample was dominated by Comment Create outliers, including one ~2.28s handler. Several deferred comment triggers classify top-level comments vs replies with WHERE comment_id = new.comment_id, while the existing primary key is ordered as (parent_comment_id, comment_id). This index gives that reverse lookup its own narrow path.

Testing

• git diff --check