feat: add node agent system with heartbeat collection, installer generation, and traffic flow telemetry

Add NodeAgent and TrafficFlow models to track agent status and network flows, implement /agents/heartbeat endpoint to receive interface counters, conntrack flows, firewall status, and nftables ruleset hash from agents, add nexafabric-agent.py Python script to collect host telemetry including VM/LXC interface hints via tap/fwbr regex matching, conntrack flow parsing with protocol/state/byte counters, and pve-firewall status checks,

frontend/src/App.tsx

@@ -11,6 +11,7 @@ import { Ipam } from "./pages/Ipam";
 import { ListPage } from "./pages/ListPage";
 import { Login } from "./pages/Login";
 import { Networks } from "./pages/Networks";
+import { Nodes } from "./pages/Nodes";
 import { Policies } from "./pages/Policies";
 import { PolicyDesigner } from "./pages/PolicyDesigner";
 import { SecurityGroups } from "./pages/SecurityGroups";
@@ -46,7 +47,7 @@ function AppRoutes() {
       <Route element={<Layout />}>
         <Route index element={<Dashboard />} />
         <Route path="clusters" element={<Clusters />} />
-        <Route path="nodes" element={<ListPage title="Nodes" subtitle="Cluster nodes, capacity, and health." path="/nodes" columns={[{ key: "name", label: "Name" }, { key: "status", label: "Status" }, { key: "cpu_count", label: "CPU" }, { key: "memory_mb", label: "Memory MB" }]} />} />
+        <Route path="nodes" element={<Nodes />} />
         <Route path="workloads" element={<Workloads />} />
         <Route path="networks" element={<Networks />} />
         <Route path="ipam" element={<Ipam />} />