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

backend/internal/repository/group_repo.go

@@ -63,7 +63,8 @@ func (r *groupRepository) Create(ctx context.Context, groupIn *service.Group) er
 		SetRequireOauthOnly(groupIn.RequireOAuthOnly).
 		SetRequirePrivacySet(groupIn.RequirePrivacySet).
 		SetDefaultMappedModel(groupIn.DefaultMappedModel).
-		SetMessagesDispatchModelConfig(groupIn.MessagesDispatchModelConfig)
+		SetMessagesDispatchModelConfig(groupIn.MessagesDispatchModelConfig).
+		SetRpmLimit(groupIn.RPMLimit)
 
 	// 设置模型路由配置
 	if groupIn.ModelRouting != nil {
@@ -130,7 +131,8 @@ func (r *groupRepository) Update(ctx context.Context, groupIn *service.Group) er
 		SetRequireOauthOnly(groupIn.RequireOAuthOnly).
 		SetRequirePrivacySet(groupIn.RequirePrivacySet).
 		SetDefaultMappedModel(groupIn.DefaultMappedModel).
-		SetMessagesDispatchModelConfig(groupIn.MessagesDispatchModelConfig)
+		SetMessagesDispatchModelConfig(groupIn.MessagesDispatchModelConfig).
+		SetRpmLimit(groupIn.RPMLimit)
 
 	// 显式处理可空字段：nil 需要 clear，非 nil 需要 set。
 	if groupIn.DailyLimitUSD != nil {