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/user_repo.go

@@ -93,6 +93,7 @@ func (r *userRepository) Create(ctx context.Context, userIn *service.User) error
 		SetSignupSource(userSignupSourceOrDefault(userIn.SignupSource)).
 		SetNillableLastLoginAt(userIn.LastLoginAt).
 		SetNillableLastActiveAt(userIn.LastActiveAt).
+		SetRpmLimit(userIn.RPMLimit).
 		Save(txCtx)
 	if err != nil {
 		return translatePersistenceError(err, nil, service.ErrEmailExists)
@@ -219,7 +220,8 @@ func (r *userRepository) Update(ctx context.Context, userIn *service.User) error
 		SetBalanceNotifyThresholdType(userIn.BalanceNotifyThresholdType).
 		SetNillableBalanceNotifyThreshold(userIn.BalanceNotifyThreshold).
 		SetBalanceNotifyExtraEmails(marshalExtraEmails(userIn.BalanceNotifyExtraEmails)).
-		SetTotalRecharged(userIn.TotalRecharged)
+		SetTotalRecharged(userIn.TotalRecharged).
+		SetRpmLimit(userIn.RPMLimit)
 	if userIn.SignupSource != "" {
 		updateOp = updateOp.SetSignupSource(userIn.SignupSource)
 	}