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/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "infrastructure-monitoring-backend",
+  "version": "1.0.0",
+  "type": "module",
+  "scripts": {
+    "start": "node server.js",
+    "dev": "node --watch server.js"
+  },
+  "dependencies": {
+    "cors": "^2.8.5",
+    "express": "^4.21.2"
+  }
+}