Use exec_query for custom SQL execution

Also align how we use query Postgres from controllers.

The change is mainly due to regular (but not reproducible) issues with the `intervalstyle` defaulting to `postgres` (instead of `iso8601`) and thereby causing issues. We're just experimenting to see whether this change resolves the issue.

app/controllers/ping_controller.rb

@@ -16,7 +16,7 @@ class PingController < ApplicationController
 
   def postgres_connected!
     # any unhandled exception leads to a HTTP 500 response.
-    return if ApplicationRecord.connection.execute('SELECT 1 as result').first['result'] == 1
+    return if ApplicationRecord.connection.exec_query('SELECT 1 as result').first['result'] == 1
 
     raise ActiveRecord::ConnectionNotEstablished
   end