Add traffic limit functionality

- Add traffic_limit field to vpn_clients table (migration 007)
- Add traffic limit management methods in VpnClient class
- Add API endpoints: set-traffic-limit, traffic-limit-status, overlimit
- Add UI for setting limits when creating clients
- Add traffic limit column in clients table with visual indicators
- Add automatic traffic limit check via cron (bin/check_traffic_limits.php)
- Add translations for traffic limits (6 languages)
- Update README with traffic limit documentation

bin/check_traffic_limits.php

@@ -0,0 +1,31 @@
+#!/usr/bin/env php
+<?php
+/**
+ * Check and disable clients that exceeded their traffic limit
+ * Run this script via cron every hour
+ */
+
+require_once __DIR__ . '/../inc/Config.php';
+require_once __DIR__ . '/../inc/DB.php';
+require_once __DIR__ . '/../inc/VpnClient.php';
+require_once __DIR__ . '/../inc/VpnServer.php';
+
+// Load config
+Config::load(__DIR__ . '/../.env');
+
+echo '[' . date('Y-m-d H:i:s') . '] Checking for clients over traffic limit...' . PHP_EOL;
+
+try {
+    $disabled = VpnClient::disableClientsOverLimit();
+    
+    if ($disabled > 0) {
+        echo '[' . date('Y-m-d H:i:s') . '] Disabled ' . $disabled . ' client(s) that exceeded traffic limit' . PHP_EOL;
+    } else {
+        echo '[' . date('Y-m-d H:i:s') . '] No clients over traffic limit' . PHP_EOL;
+    }
+} catch (Exception $e) {
+    echo '[' . date('Y-m-d H:i:s') . '] ERROR: ' . $e->getMessage() . PHP_EOL;
+    exit(1);
+}
+
+exit(0);