Add Node.js backend API with mock providers for all 15 vendors

Express app on port 3000 with /api/status and /api/health endpoints.
Polls all providers every 2 minutes and caches results in memory.
Each vendor is a self-contained ESM module with 10s timeout and
graceful failure handling. Mock data matches existing frontend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/providers/local-infrastructure.js

@@ -0,0 +1,10 @@
+export const name = "Local Infrastructure";
+
+export async function checkStatus() {
+  return {
+    name,
+    status: "operational",
+    message: "Firewall uptime 42d. WAN throughput nominal.",
+    lastUpdated: new Date().toISOString(),
+  };
+}