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

install/servermanager-agent.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=ServerManager Monitoring Agent
+Documentation=https://github.com/servermanager
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/servermanager-agent
+Restart=always
+RestartSec=10
+StartLimitIntervalSec=60
+StartLimitBurst=5
+NoNewPrivileges=yes
+ProtectSystem=full
+ProtectHome=yes
+PrivateDevices=yes
+PrivateTmp=yes
+
+[Install]
+WantedBy=multi-user.target