refactor: open caqti module on query and mutation

server/server.ml

 module type DB = Caqti_lwt.CONNECTION
-
 module T = Caqti_type
-open Caqti_request.Infix
 
 let getenv_with_default key fallback =
   Option.value (Sys.getenv_opt key) ~default:fallback
@@ -16,13 +14,17 @@ let setup_sql_database =
        postgres_host postgres_db)
 
 let list_comments =
-  let query = (T.unit ->* T.(tup2 int string)) "SELECT id, text FROM comment" in
+  let query =
+    let open Caqti_request.Infix in
+    (T.unit ->* T.(tup2 int string)) "SELECT id, text FROM comment"
+  in
   fun (module Db : DB) ->
     let%lwt comments_or_error = Db.collect_list query () in
     Caqti_lwt.or_fail comments_or_error
 
 let add_comment =
   let mutation =
+    let open Caqti_request.Infix in
     (T.string ->. T.unit) "INSERT INTO comment (text) VALUES ($1)"
   in
   fun text (module Db : DB) ->