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/server.js

@@ -0,0 +1,59 @@
+import express from "express";
+import cors from "cors";
+import { providers } from "./providers/index.js";
+
+const app = express();
+const PORT = 3000;
+const POLL_INTERVAL = 2 * 60 * 1000; // 2 minutes
+const PROVIDER_TIMEOUT = 10 * 1000; // 10 seconds
+
+app.use(cors());
+
+let cachedStatuses = [];
+
+function safeCheck(provider) {
+  return Promise.race([
+    provider.checkStatus(),
+    new Promise((_, reject) =>
+      setTimeout(() => reject(new Error("Timeout")), PROVIDER_TIMEOUT)
+    ),
+  ]).catch((err) => ({
+    name: provider.name,
+    status: "unknown",
+    message: `Check failed: ${err.message}`,
+    lastUpdated: new Date().toISOString(),
+  }));
+}
+
+async function pollAll() {
+  const results = await Promise.allSettled(providers.map(safeCheck));
+  cachedStatuses = results.map((r) =>
+    r.status === "fulfilled"
+      ? r.value
+      : {
+          name: "Unknown",
+          status: "unknown",
+          message: "Provider check failed.",
+          lastUpdated: new Date().toISOString(),
+        }
+  );
+  console.log(
+    `[${new Date().toISOString()}] Polled ${cachedStatuses.length} providers`
+  );
+}
+
+app.get("/api/status", (_req, res) => {
+  res.json(cachedStatuses);
+});
+
+app.get("/api/health", (_req, res) => {
+  res.json({ ok: true, providers: providers.length });
+});
+
+// Poll immediately, then on interval
+await pollAll();
+setInterval(pollAll, POLL_INTERVAL);
+
+app.listen(PORT, () => {
+  console.log(`Backend listening on http://localhost:${PORT}`);
+});