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/ServerController.php

@@ -255,6 +255,40 @@ class ServerController
         ]);
     }
 
+    public function agentInstall(int $id): void
+    {
+        $server = $this->requireServerAccess($id, 'manager');
+
+        $agentService = new \ServerManager\Services\AgentService();
+        $result = $agentService->install($id);
+
+        $this->view->json($result);
+    }
+
+    public function agentUninstall(int $id): void
+    {
+        $server = $this->requireServerAccess($id, 'manager');
+
+        $agentService = new \ServerManager\Services\AgentService();
+        $result = $agentService->uninstall($id);
+
+        $this->view->json($result);
+    }
+
+    public function agentRegenerateKey(int $id): void
+    {
+        $this->requireServerAccess($id, 'manager');
+
+        $newKey = $this->serverModel->regenerateAgentKey($id);
+
+        $this->auditService->log('agent_key_regenerated', 'server', $id);
+
+        $this->view->json([
+            'success' => true,
+            'agent_key' => $newKey,
+        ]);
+    }
+
     public function testConnection(int $id): void
     {
         $server = $this->requireServerAccess($id, 'viewer');