feat: migrate payment provider to easy-pay, add order history and refund support

- Replace zpay with easy-pay payment provider (new lib/easy-pay/ module)
- Add order history page for users (pay/orders)
- Add GET /api/orders/my endpoint to list user's own orders
- Add GET /api/users/[id] endpoint for sub2api user lookup
- Add order status tracking module (lib/order/status.ts)
- Update config to support easy-pay credentials (merchant ID, key, gateway)
- Update PaymentForm and PaymentQRCode components for easy-pay flow
- Update pay page and admin page with new order management UI
- Update order service to support easy-pay, cancellation, and refund

.gitignore

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+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/versions
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.pnpm-debug.log*
+
+# env files (can opt-in for committing if needed)
+.env*
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts
+
+# third-party source code (local reference only)
+/third-party