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/Services/MonitoringService.php

@@ -111,7 +111,7 @@ class MonitoringService
         return trim(str_replace('up ', '', $result['output']));
     }
 
-    private function saveMetrics(array $metrics): void
+    public function saveMetrics(array $metrics): void
     {
         $now = date('Y-m-d H:i:s');