feat: Implement server and client metrics collection and monitoring

- Added a new PHP script for collecting server metrics every 30 seconds.
- Created a ServerMonitoring class to handle metrics collection for CPU, RAM, Disk, and Network.
- Introduced database tables for storing server and client metrics.
- Updated server view template to display real-time metrics using Chart.js.
- Added translations for monitoring UI elements.
- Created a new monitoring template for detailed server metrics visualization.
- Implemented client speed tracking and display in the monitoring UI.

migrations/009_add_server_metrics.sql

@@ -0,0 +1,31 @@
+-- Add server metrics tables
+-- This migration adds functionality to store and display server monitoring data
+
+-- Server metrics (CPU, RAM, Disk, Network)
+CREATE TABLE IF NOT EXISTS server_metrics (
+  id BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+  server_id INT UNSIGNED NOT NULL,
+  cpu_percent DECIMAL(5,2) NULL COMMENT 'CPU usage percentage',
+  ram_used_mb INT UNSIGNED NULL COMMENT 'RAM used in MB',
+  ram_total_mb INT UNSIGNED NULL COMMENT 'Total RAM in MB',
+  disk_used_gb DECIMAL(10,2) NULL COMMENT 'Disk used in GB',
+  disk_total_gb DECIMAL(10,2) NULL COMMENT 'Total disk in GB',
+  network_rx_mbps DECIMAL(10,2) NULL COMMENT 'Network receive speed in Mbps',
+  network_tx_mbps DECIMAL(10,2) NULL COMMENT 'Network transmit speed in Mbps',
+  collected_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  INDEX idx_server_time (server_id, collected_at),
+  FOREIGN KEY (server_id) REFERENCES vpn_servers(id) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+-- Client traffic metrics (speed tracking)
+CREATE TABLE IF NOT EXISTS client_metrics (
+  id BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+  client_id INT UNSIGNED NOT NULL,
+  bytes_sent BIGINT UNSIGNED DEFAULT 0 COMMENT 'Bytes sent at this moment',
+  bytes_received BIGINT UNSIGNED DEFAULT 0 COMMENT 'Bytes received at this moment',
+  speed_up_kbps DECIMAL(10,2) NULL COMMENT 'Upload speed in Kbps',
+  speed_down_kbps DECIMAL(10,2) NULL COMMENT 'Download speed in Kbps',
+  collected_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  INDEX idx_client_time (client_id, collected_at),
+  FOREIGN KEY (client_id) REFERENCES vpn_clients(id) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;