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/src/core/config/settings.py

@@ -228,6 +228,18 @@ class PointsPolicySettings(BaseModel):
         return self
 
 
+class AppleIapSettings(BaseModel):
+    bundle_id: str = Field(default="com.meeyao.qianwen", min_length=1)
+    root_cert_url: str = "https://www.apple.com/certificateauthority/AppleIncRootCertificate.cer"
+    jws_x5c_cert_url: str = "https://api.storekit.itunes.apple.com/v1/verificationKeys"
+    server_api_issuer_id: str | None = None
+    server_api_key_id: str | None = None
+    server_api_private_key: SecretStr | None = None
+    sandbox_tester_email: str | None = None
+    sandbox_tester_password: SecretStr | None = None
+    server_notifications_url: str | None = None
+
+
 def _resolve_env_file() -> str:
     current = Path(__file__).resolve()
     for parent in [current, *current.parents]:
@@ -271,6 +283,7 @@ class Settings(BaseSettings):
     taskiq: TaskiqSettings = Field(default_factory=TaskiqSettings)
     agent_runtime: AgentRuntimeSettings = Field(default_factory=AgentRuntimeSettings)
     points_policy: PointsPolicySettings = Field(default_factory=PointsPolicySettings)
+    apple_iap: AppleIapSettings = Field(default_factory=AppleIapSettings)
     feedback_report: FeedbackReportSettings = Field(
         default_factory=FeedbackReportSettings
     )