feat: implement thin vertical slice of system-generated notifications (#13537)

coderd/database/queries/notifications.sql

@@ -125,3 +125,6 @@ WHERE id IN
        FROM notification_messages AS nested
        WHERE nested.updated_at < NOW() - INTERVAL '7 days');
 
+-- name: GetNotificationMessagesByStatus :many
+SELECT * FROM notification_messages WHERE status = @status LIMIT sqlc.arg('limit')::int;
+