chore: fix performance of a couple of queries (CM-1008)

[themarolt]

Note

Medium Risk
Rewrites the SQL used to count and list member/organization merge suggestions; while intended as a performance optimization, it can change result counts/order if assumptions about uniqueness or segment membership differ.

Overview
Improves performance of merge-suggestion queries for members and organizations by replacing segment-table joins with EXISTS checks and avoiding unnecessary joins unless a displayName filter is applied.

For members, the count query switches to COUNT(*) and the list query drops DISTINCT, relying on the memberToMerge row set plus EXISTS-based segment filtering.

For organizations, the count query removes the previous cte and counts distinct symmetric pairs via a hash, and the list query replaces segment joins with EXISTS plus scalar subselects to fetch a matching primary/secondary segment id.

^{Written by Cursor Bugbot for commit 468c588. This will update automatically on new commits. Configure here.}