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/007_add_traffic_limit.sql

@@ -0,0 +1,6 @@
+-- Add traffic limit field to vpn_clients table
+-- This migration adds traffic limit functionality to clients
+
+ALTER TABLE vpn_clients 
+ADD COLUMN traffic_limit BIGINT UNSIGNED NULL COMMENT 'Traffic limit in bytes (NULL = unlimited)' AFTER traffic_received,
+ADD INDEX idx_traffic_limit (traffic_limit);