server: fetch search summaries in parallel

server/sql/need-search-summary.sql

@@ -3,6 +3,7 @@ SELECT
     link,
     clean_summary
 FROM
+-- Remoe tablesample when db sufficiently indexed
     post AS p TABLESAMPLE SYSTEM (.1)
 INNER JOIN feed AS f ON p.site = f.slug -- necessary to weed out nzb posts
 WHERE search_summary IS NULL;