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/handler/dto/mappers.go

@@ -29,6 +29,7 @@ func UserFromServiceShallow(u *service.User) *User {
 		BalanceNotifyThreshold:     u.BalanceNotifyThreshold,
 		BalanceNotifyExtraEmails:   NotifyEmailEntriesFromService(u.BalanceNotifyExtraEmails),
 		TotalRecharged:             u.TotalRecharged,
+		RPMLimit:                   u.RPMLimit,
 	}
 }
 
@@ -184,6 +185,7 @@ func groupFromServiceBase(g *service.Group) Group {
 		AllowMessagesDispatch:           g.AllowMessagesDispatch,
 		RequireOAuthOnly:                g.RequireOAuthOnly,
 		RequirePrivacySet:               g.RequirePrivacySet,
+		RPMLimit:                        g.RPMLimit,
 		CreatedAt:                       g.CreatedAt,
 		UpdatedAt:                       g.UpdatedAt,
 	}