feat: push-based monitoring with remote agent

- Add database migration 006 for agent_key, agent_installed, agent_install_path, agent_last_seen_at columns
- Add Server model methods: findByAgentKey, generateAgentKey, regenerateAgentKey, updateAgentLastSeen, setAgentInstalled
- Auto-generate agent_key on server creation
- Create AgentService for SSH-based install/uninstall of remote agent
- Create bin/agent.sh - bash agent script for remote servers
- Create install/agent-install.sh and install/agent-uninstall.sh
- Create install/servermanager-agent.service systemd unit
- Add POST /api/metrics/push endpoint (auth via agent_key)
- Add web routes for agent install/uninstall/regenerate-key
- Add agent status section to server detail view with install/uninstall modals
- Make MonitoringService::saveMetrics() public for reuse

src/Controllers/ApiController.php

@@ -114,6 +114,8 @@ class ApiController
         $monitoringService = new MonitoringService();
         $metrics = $monitoringService->getLatestMetrics($id);
 
+        unset($server['agent_key'], $server['ssh_password'], $server['ssh_key']);
+
         $this->view->json([
             'success' => true,
             'data' => [
@@ -296,6 +298,43 @@ class ApiController
         ]);
     }
 
+    public function pushMetrics(): void
+    {
+        $input = json_decode(file_get_contents('php://input'), true);
+
+        if (!$input || empty($input['agent_key'])) {
+            $this->view->json(['error' => 'agent_key is required'], 401);
+        }
+
+        $serverModel = new Server();
+        $server = $serverModel->findByAgentKey($input['agent_key']);
+
+        if (!$server || !$server['is_active']) {
+            $this->view->json(['error' => 'Invalid agent key'], 401);
+        }
+
+        $cpu = isset($input['cpu']) ? max(0, min(100, (float) $input['cpu'])) : 0;
+        $ram = isset($input['ram']) ? max(0, min(100, (float) $input['ram'])) : 0;
+        $disk = isset($input['disk']) ? max(0, min(100, (float) $input['disk'])) : 0;
+        $load = (float) ($input['load'] ?? 0);
+        $uptime = (string) ($input['uptime'] ?? '');
+
+        $monitoring = new MonitoringService();
+        $monitoring->saveMetrics([
+            'server_id' => (int) $server['id'],
+            'status' => 'online',
+            'cpu_usage' => $cpu,
+            'ram_usage' => $ram,
+            'disk_usage' => $disk,
+            'load_average' => $load,
+            'uptime' => $uptime,
+        ]);
+
+        $serverModel->updateAgentLastSeen((int) $server['id']);
+
+        $this->view->json(['success' => true]);
+    }
+
     public function refreshAllMetrics(): void
     {
         $user = $this->authenticateRequest();