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/agent-uninstall.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# ServerManager Agent Uninstaller
+set -e
+
+AGENT_BIN="/usr/local/bin/servermanager-agent"
+CONFIG_DIR="/etc/servermanager"
+SERVICE_FILE="/etc/systemd/system/servermanager-agent.service"
+
+echo "[1/3] Stopping and disabling service..."
+systemctl stop servermanager-agent 2>/dev/null || true
+systemctl disable servermanager-agent 2>/dev/null || true
+systemctl daemon-reload
+
+echo "[2/3] Removing files..."
+rm -f "$AGENT_BIN"
+rm -rf "$CONFIG_DIR"
+rm -f "$SERVICE_FILE"
+
+echo "[3/3] Reloading systemd..."
+systemctl daemon-reload
+
+echo "ServerManager Agent uninstalled successfully."