Let the login throttle see real client IPs behind Cloudflare

Behind a proxy REMOTE_ADDR is the proxy, so the per-IP limit would lump
every visitor together. New optional CLIENT_IP_HEADER in config.php
(e.g. 'CF-Connecting-IP') tells the throttle which header carries the
real address; invalid or absent values fall back to REMOTE_ADDR, and
configs that predate the setting behave as before. Only for origins
unreachable except through the proxy — documented in the README, which
also covers the mod_remoteip/real_ip alternative.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SePf61iazhCLJBefdRLBDm

includes/auth.php

@@ -102,6 +102,25 @@ const LOGIN_FAIL_WINDOW   = 900;  // seconds
 const LOGIN_MAX_USER_FAILS = 5;
 const LOGIN_MAX_IP_FAILS   = 20;
 
+/**
+ * The client's IP for the throttle. Behind a proxy that hides the visitor
+ * (Cloudflare), REMOTE_ADDR is the proxy — set CLIENT_IP_HEADER in
+ * config.php to the header carrying the real address instead. Optional and
+ * off by default: installs whose config.php predates the setting keep using
+ * REMOTE_ADDR.
+ */
+function clientIp(): string
+{
+    if (defined('CLIENT_IP_HEADER') && CLIENT_IP_HEADER !== '') {
+        $key = 'HTTP_' . strtoupper(str_replace('-', '_', CLIENT_IP_HEADER));
+        $ip  = trim(explode(',', $_SERVER[$key] ?? '')[0]);
+        if ($ip !== '' && filter_var($ip, FILTER_VALIDATE_IP)) {
+            return $ip;
+        }
+    }
+    return $_SERVER['REMOTE_ADDR'] ?? '';
+}
+
 /** Seconds until this username/IP may try again — 0 means go ahead. */
 function loginRetryAfter(string $username, string $ip): int
 {