Add reposts user created-at index

[raymondjacobson]

Summary

• add a concurrent partial covering index for /v1/users/:id/reposts pagination
• index active reposts by (user_id, created_at DESC) and include repost_type / repost_item_id
• keep endpoint SQL unchanged; this codifies the index that was manually applied and validated on prod

DB evidence

Live read-only analysis showed the existing plan could scan global reposts_new_created_at_idx and discard millions of unrelated rows for users whose reposts were not globally recent.

For heavy user 279392 before the index:

• LIMIT 10 OFFSET 0: ~7.7s cold, skipped ~2.65M unrelated rows
• LIMIT 100 OFFSET 0: ~1.35s warm, skipped ~4.51M unrelated rows
• LIMIT 100 OFFSET 1000: ~1.30s warm, skipped ~4.52M unrelated rows

After applying reposts_user_created_at_active_idx, the planner used Index Only Scan using reposts_user_created_at_active_idx:

• LIMIT 10 OFFSET 0: ~6.5ms
• LIMIT 100 OFFSET 0: ~6.4ms
• LIMIT 100 OFFSET 1000: ~35ms
• fresh-heavy user 714646433 LIMIT 10 OFFSET 0: ~4.9ms

The live index was valid/ready/live and ~226 MB.

Test

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