feat(rpm): RPM 限流模块优化

P0:
- rpm_override 嵌入 Auth Cache Snapshot，消除每请求 DB 查询 (snapshot v6→v7)
- 429 RPM 响应返回 Retry-After 头（当前分钟剩余秒数）

P1:
- ClearAll 按钮直连 DELETE API，带 loading 防重复
- 新增 GET /admin/users/:id/rpm-status 管理员 RPM 用量查询端点

优化:
- checkRPM 从级联互斥改为并行取最严，user.rpm_limit 作为全局硬上限始终生效
- Override/Group 变更后自动失效 auth cache
- fail-open 语义不变，Redis 故障不阻塞业务

frontend/src/api/admin/settings.ts

@@ -309,6 +309,7 @@ export interface SystemSettings {
   // Default settings
   default_balance: number;
   default_concurrency: number;
+  default_user_rpm_limit: number;
   default_subscriptions: DefaultSubscriptionSetting[];
   auth_source_default_email_balance?: number;
   auth_source_default_email_concurrency?: number;
@@ -482,6 +483,7 @@ export interface UpdateSettingsRequest {
   totp_enabled?: boolean; // TOTP 双因素认证
   default_balance?: number;
   default_concurrency?: number;
+  default_user_rpm_limit?: number;
   default_subscriptions?: DefaultSubscriptionSetting[];
   auth_source_default_email_balance?: number;
   auth_source_default_email_concurrency?: number;