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/.env.example

@@ -1,11 +1,17 @@
+# Server
+SERVER_PORT=3001
+
+# Hono ERP API (museum sales data)
+ERP_API_URL=https://hono-erp.azurewebsites.net
+ERP_API_CODE=your-api-function-key
+ERP_USERNAME=your-username
+ERP_PASSWORD=your-password
+
 # Salla OAuth Credentials (from Salla Partners dashboard)
 SALLA_CLIENT_ID=your_client_id_here
 SALLA_CLIENT_SECRET=your_client_secret_here
 SALLA_REDIRECT_URI=http://localhost:3001/auth/callback
 
-# Server port
-SALLA_SERVER_PORT=3001
-
 # After OAuth, these will be populated automatically
 # SALLA_ACCESS_TOKEN=
 # SALLA_REFRESH_TOKEN=