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/types/index.ts

@@ -87,6 +87,7 @@ export interface User {
   role: 'admin' | 'user' // User role for authorization
   balance: number // User balance for API usage
   concurrency: number // Allowed concurrent requests
+  rpm_limit?: number // User-level RPM cap (0 = unlimited); effective as fallback when group has no rpm_limit
   status: 'active' | 'disabled' // Account status
   allowed_groups: number[] | null // Allowed group IDs (null = all non-exclusive groups)
   balance_notify_enabled: boolean
@@ -453,6 +454,7 @@ export interface Group {
   description: string | null
   platform: GroupPlatform
   rate_multiplier: number
+  rpm_limit?: number // Group-level RPM cap (0 = unlimited); overrides user-level rpm_limit when set
   is_exclusive: boolean
   status: 'active' | 'inactive'
   subscription_type: SubscriptionType