feat(monitoring): add node/tunnel metrics, service monitors, and health checks

- Add NodeMetric/TunnelMetric/ServiceMonitor models and repository methods
- Implement metrics ingestion service with per-minute bucket aggregation
- Add health checker for node connectivity monitoring
- Wire node metrics from WebSocket SystemInfo messages
- Add tunnel metrics ingestion from flow upload endpoint
- Create monitoring REST API endpoints for nodes, tunnels, services
- Implement service monitor CRUD and execution (TCP/ICMP checks)
- Add MonitorPermission for non-admin access control
- Create frontend monitor page with node/tunnel/service views
- Add tunnel metrics ingestion from agent flow reports
- Include schema migration for tunnel_metric unique index
- Fix tunnel entry port conflict validation to use transaction

Entire-Checkpoint: 030821a7c8e3

vite-frontend/src/App.tsx

@@ -4,6 +4,7 @@ import { useEffect } from "react";
 import IndexPage from "@/pages/index";
 import ChangePasswordPage from "@/pages/change-password";
 import DashboardPage from "@/pages/dashboard";
+import MonitorPage from "@/pages/monitor";
 import ForwardPage from "@/pages/forward";
 import TunnelPage from "@/pages/tunnel";
 import NodePage from "@/pages/node";
@@ -122,6 +123,14 @@ function App() {
         }
         path="/dashboard"
       />
+      <Route
+        element={
+          <ProtectedRoute>
+            <MonitorPage />
+          </ProtectedRoute>
+        }
+        path="/monitor"
+      />
       <Route
         element={
           <ProtectedRoute>