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/runtime/runtime.go

@@ -595,6 +595,10 @@ func init() {
 	groupDescMessagesDispatchModelConfig := groupFields[26].Descriptor()
 	// group.DefaultMessagesDispatchModelConfig holds the default value on creation for the messages_dispatch_model_config field.
 	group.DefaultMessagesDispatchModelConfig = groupDescMessagesDispatchModelConfig.Default.(domain.OpenAIMessagesDispatchModelConfig)
+	// groupDescRpmLimit is the schema descriptor for rpm_limit field.
+	groupDescRpmLimit := groupFields[27].Descriptor()
+	// group.DefaultRpmLimit holds the default value on creation for the rpm_limit field.
+	group.DefaultRpmLimit = groupDescRpmLimit.Default.(int)
 	idempotencyrecordMixin := schema.IdempotencyRecord{}.Mixin()
 	idempotencyrecordMixinFields0 := idempotencyrecordMixin[0].Fields()
 	_ = idempotencyrecordMixinFields0
@@ -1575,6 +1579,10 @@ func init() {
 	userDescTotalRecharged := userFields[18].Descriptor()
 	// user.DefaultTotalRecharged holds the default value on creation for the total_recharged field.
 	user.DefaultTotalRecharged = userDescTotalRecharged.Default.(float64)
+	// userDescRpmLimit is the schema descriptor for rpm_limit field.
+	userDescRpmLimit := userFields[19].Descriptor()
+	// user.DefaultRpmLimit holds the default value on creation for the rpm_limit field.
+	user.DefaultRpmLimit = userDescRpmLimit.Default.(int)
 	userallowedgroupFields := schema.UserAllowedGroup{}.Fields()
 	_ = userallowedgroupFields
 	// userallowedgroupDescCreatedAt is the schema descriptor for created_at field.