feat: 实现 iOS Apple Pay 内购支付功能

前端:
- 集成 in_app_purchase 插件，实现 IAP 支付流程
- 添加支付模块 (payments/) 处理产品获取、购买、验证
- 积分中心页面集成 Apple Pay 购买入口
- 设置页面重构: 关于/隐私/协议直接展示，删除 legal_center 子页面
- 修复欢迎引导页滚动检测阈值问题
- 修复解卦结果页 iOS 侧滑返回手势被阻止的问题
- 邀请码绑定按钮临时禁用（待后端实现）

后端:
- 新增 apple_iap_transactions 表记录交易
- 实现 Apple 服务器端验证 (App Store Server API)
- 支付成功后自动发放积分
- 支持 Sandbox/Production 环境切换
- 添加退款处理和交易状态机

协议:
- 更新积分流水协议，支持 purchase/refund 类型
- 新增 PAYMENT_* 错误码

backend/tests/integration/payments/test_verify_flow.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+"""
+Integration tests for Apple IAP payment verify flow.
+
+Prerequisite: backend must be running via `./infra/scripts/app.sh restart`.
+These tests hit the live HTTP API against the test database.
+"""
+
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_verify_endpoint_returns_401_without_auth() -> None:
+    import httpx
+
+    base_url = "http://localhost:8000"
+    try:
+        async with httpx.AsyncClient(base_url=base_url, timeout=5) as client:
+            response = await client.post(
+                "/api/v1/payments/apple/transactions/verify",
+                json={
+                    "productCode": "basic_pack",
+                    "appStoreProductId": "com.meeyao.qianwen.basic_pack",
+                    "transactionId": "0000000000000001",
+                    "signedTransactionInfo": "fake_jws",
+                },
+            )
+            assert response.status_code in (401, 403)
+    except httpx.ConnectError:
+        pytest.skip("Backend not running, skipping integration test")
+
+
+@pytest.mark.asyncio
+async def test_notifications_endpoint_returns_200() -> None:
+    import httpx
+
+    base_url = "http://localhost:8000"
+    try:
+        async with httpx.AsyncClient(base_url=base_url, timeout=5) as client:
+            response = await client.post(
+                "/api/v1/payments/apple/notifications",
+                json={"signedPayload": ""},
+            )
+            assert response.status_code == 200
+    except httpx.ConnectError:
+        pytest.skip("Backend not running, skipping integration test")