Port the code to PHP 8.5.

Replace the short open tags and the removed functions (mcrypt, mysql_error,
salt-less crypt). Decode the reCAPTCHA reply as JSON, turn off mysqli
exceptions, drop stripslashes(), and default missing input values.
Fix the misplaced continue in prune_images().

open_db.lib.php

@@ -1,4 +1,4 @@
-<?
+<?php
 require_once(__DIR__ . '/config.lib.php');
 global $db;
 function open_db($state) {
@@ -9,6 +9,8 @@ function open_db($state) {
       $view_user = config('DB_USER');
       $view_pass = config('DB_PASS');
       $db_name   = config('DB_NAME');
+      // PHP 8.1+ throws exceptions by default; keep the old return-value checks working
+      mysqli_report(MYSQLI_REPORT_OFF);
       $db = new mysqli($hostname, $view_user, $view_pass, $db_name);
       if ($db->connect_error) {
         die ("Unable to connect to database");