perf(db): add saves favorites ordering index

ddl/migrations/0220_saves_user_track_current_blocknumber_idx.sql

@@ -0,0 +1,17 @@
+-- Supports /v1/users/:id/favorites pagination:
+--
+--   WHERE user_id = ?
+--     AND is_delete = false
+--     AND is_current = true
+--     AND save_type = 'track'
+--   ORDER BY blocknumber, save_item_id DESC
+--
+-- The existing saves_user_idx is ordered by save_item_id before is_delete and
+-- cannot satisfy the endpoint's blocknumber ordering, which showed up in prod
+-- pg_stat_statements with high total temp I/O.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS saves_user_track_current_blocknumber_idx
+    ON public.saves USING btree (user_id, blocknumber, save_item_id DESC)
+    INCLUDE (created_at)
+    WHERE save_type = 'track'
+      AND is_current = true
+      AND is_delete = false;