feat: convert server to TypeScript + add ERP API proxy

- Migrate server/index.js → modular TS structure (config, routes, services)
- Add ERP proxy: GET /api/erp/sales proxies Hono ERP API with server-side auth
- JWT token cached server-side, auto-refreshes on 401
- ERP credentials stay server-side only (no VITE_ prefix)
- Vite dev proxy routes /api/erp → localhost:3001
- Preserve existing Salla OAuth integration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/routes/erp.ts

@@ -0,0 +1,34 @@
+import { Router, Request, Response } from 'express';
+import { fetchSales, isConfigured } from '../services/erpClient';
+
+const router = Router();
+
+// GET /api/erp/sales?startDate=2025-01-01T00:00:00&endDate=2025-01-31T00:00:00
+router.get('/sales', async (req: Request, res: Response) => {
+  if (!isConfigured()) {
+    res.status(503).json({ error: 'ERP API not configured on server' });
+    return;
+  }
+
+  const { startDate, endDate } = req.query;
+
+  if (!startDate || !endDate) {
+    res.status(400).json({ error: 'startDate and endDate query params required' });
+    return;
+  }
+
+  try {
+    const data = await fetchSales(startDate as string, endDate as string);
+    res.json(data);
+  } catch (err) {
+    console.error('ERP fetch error:', (err as Error).message);
+    res.status(502).json({ error: 'Failed to fetch from ERP API', details: (err as Error).message });
+  }
+});
+
+// GET /api/erp/status
+router.get('/status', (_req: Request, res: Response) => {
+  res.json({ configured: isConfigured() });
+});
+
+export default router;