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/server/routes/admin.go

@@ -221,6 +221,7 @@ func registerUserManagementRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		users.GET("/:id/usage", h.Admin.User.GetUserUsage)
 		users.GET("/:id/balance-history", h.Admin.User.GetBalanceHistory)
 		users.POST("/:id/replace-group", h.Admin.User.ReplaceGroup)
+		users.GET("/:id/rpm-status", h.Admin.User.GetUserRPMStatus)
 
 		// User attribute values
 		users.GET("/:id/attributes", h.Admin.UserAttribute.GetUserAttributes)
@@ -244,6 +245,8 @@ func registerGroupRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		groups.GET("/:id/rate-multipliers", h.Admin.Group.GetGroupRateMultipliers)
 		groups.PUT("/:id/rate-multipliers", h.Admin.Group.BatchSetGroupRateMultipliers)
 		groups.DELETE("/:id/rate-multipliers", h.Admin.Group.ClearGroupRateMultipliers)
+		groups.PUT("/:id/rpm-overrides", h.Admin.Group.BatchSetGroupRPMOverrides)
+		groups.DELETE("/:id/rpm-overrides", h.Admin.Group.ClearGroupRPMOverrides)
 		groups.GET("/:id/api-keys", h.Admin.Group.GetGroupAPIKeys)
 	}
 }