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/ent/group.go

@@ -79,6 +79,8 @@ type Group struct {
 	DefaultMappedModel string `json:"default_mapped_model,omitempty"`
 	// OpenAI Messages 调度模型配置：按 Claude 系列/精确模型映射到目标 GPT 模型
 	MessagesDispatchModelConfig domain.OpenAIMessagesDispatchModelConfig `json:"messages_dispatch_model_config,omitempty"`
+	// 分组 RPM 上限，0 表示不限制；设置后接管该分组用户的限流
+	RpmLimit int `json:"rpm_limit,omitempty"`
 	// Edges holds the relations/edges for other nodes in the graph.
 	// The values are being populated by the GroupQuery when eager-loading is set.
 	Edges        GroupEdges `json:"edges"`
@@ -191,7 +193,7 @@ func (*Group) scanValues(columns []string) ([]any, error) {
 			values[i] = new(sql.NullBool)
 		case group.FieldRateMultiplier, group.FieldDailyLimitUsd, group.FieldWeeklyLimitUsd, group.FieldMonthlyLimitUsd, group.FieldImagePrice1k, group.FieldImagePrice2k, group.FieldImagePrice4k:
 			values[i] = new(sql.NullFloat64)
-		case group.FieldID, group.FieldDefaultValidityDays, group.FieldFallbackGroupID, group.FieldFallbackGroupIDOnInvalidRequest, group.FieldSortOrder:
+		case group.FieldID, group.FieldDefaultValidityDays, group.FieldFallbackGroupID, group.FieldFallbackGroupIDOnInvalidRequest, group.FieldSortOrder, group.FieldRpmLimit:
 			values[i] = new(sql.NullInt64)
 		case group.FieldName, group.FieldDescription, group.FieldStatus, group.FieldPlatform, group.FieldSubscriptionType, group.FieldDefaultMappedModel:
 			values[i] = new(sql.NullString)
@@ -414,6 +416,12 @@ func (_m *Group) assignValues(columns []string, values []any) error {
 					return fmt.Errorf("unmarshal field messages_dispatch_model_config: %w", err)
 				}
 			}
+		case group.FieldRpmLimit:
+			if value, ok := values[i].(*sql.NullInt64); !ok {
+				return fmt.Errorf("unexpected type %T for field rpm_limit", values[i])
+			} else if value.Valid {
+				_m.RpmLimit = int(value.Int64)
+			}
 		default:
 			_m.selectValues.Set(columns[i], values[i])
 		}
@@ -599,6 +607,9 @@ func (_m *Group) String() string {
 	builder.WriteString(", ")
 	builder.WriteString("messages_dispatch_model_config=")
 	builder.WriteString(fmt.Sprintf("%v", _m.MessagesDispatchModelConfig))
+	builder.WriteString(", ")
+	builder.WriteString("rpm_limit=")
+	builder.WriteString(fmt.Sprintf("%v", _m.RpmLimit))
 	builder.WriteByte(')')
 	return builder.String()
 }