Add comments user pagination index

[raymondjacobson]

Summary

• add a concurrent partial covering index for /v1/users/:id/comments pagination
• remove an unused comment_threads join from the user-comments id page query
• keep hydration/filtering behavior in FullComments unchanged

DB evidence

Read-only investigation against discovery-1-replica showed this route is currently dominating the read replica.

Live 60-second pg_stat_statements delta:

• SELECT comment_id as id FROM comments ... WHERE user_id = $1 ... ORDER BY comments.created_at DESC LIMIT/OFFSET
• 1,610 calls in 60s
• ~1,666,937 ms accumulated DB execution time
• ~1,035 ms mean execution time

Post-rollout 30-second delta still showed ongoing pressure:

• 954 calls in 30s
• ~986,306 ms accumulated DB execution time
• ~1,034 ms mean execution time

Representative EXPLAIN (ANALYZE, BUFFERS) on the replica:

• user 527480232, LIMIT 10 OFFSET 0: ~1.07s
• user 20, LIMIT 10 OFFSET 0: ~1.05s
• user 527480232, LIMIT 10 OFFSET 1000: ~1.08s

The plan scans the full comments table, hashes comment_threads, sorts, and returns only a page of ids. Catalog check showed comments only had comments_pkey and comments_blocknumber_idx, with no index matching user_id + Track + active + created_at.

Bridge logs also show the live traffic source is IP 162.220.232.107 using app_name=TEMPO, TEMPO_sync, TEMPO_engage, and TEMPO_analytics against /v1/users/:userId/comments.

Test

• go test ./api -run 'TestUserComments|Test200UnAuthed' -count=1