feat: Admin API enhancements - Create/Update App, Per-App Stripe, One-Time Payments

Closes #305

Features implemented:
1. Create & Update App via Admin API
   - POST /v1/admin/apps - Create new app with access token auth
   - PATCH /v1/admin/apps/:appId - Update app properties (paymentsMode, paymentRequired, etc.)

2. Independent Stripe per App
   - Each app can have its own Stripe account connected
   - POST /v1/admin/apps/:appId/stripe/connection/:authorizationCode
   - DELETE /v1/admin/apps/:appId/stripe/connection
   - Per-app Stripe account takes precedence over admin's Stripe account

3. One-Time Payment Support
   - New WebSocket action: PurchaseOneTime
   - User purchases stored in 'purchases' array
   - Webhook handling for checkout.session.completed (payment mode)
   - Refund handling via charge.refunded webhook

Changes:
- app.js: Added Admin API endpoints for app management
- user.js: Added one-time payment support and per-app Stripe account support
- stripe.js: Added webhook handlers for one-time payments
- server.js: Added new API routes for Admin API

Author: HuiNeng6