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

migrations/001_init.sql

@@ -205,6 +205,9 @@ INSERT INTO translations (language_code, translation_key, translation_value) VAL
 ('en', 'clients.sync_stats', 'Sync Stats'),
 ('en', 'clients.title', 'Clients'),
 ('en', 'clients.traffic', 'Traffic'),
+('en', 'clients.traffic_limit', 'Traffic Limit'),
+('en', 'clients.unlimited', 'Unlimited'),
+('en', 'clients.overlimit', 'Over Limit'),
 ('en', 'backups.title', 'Server Backups'),
 ('en', 'backups.create', 'Create Backup'),
 ('en', 'backups.restore', 'Restore'),