feat(acceptance): 增加生产同构媒体压力验收模式

引入动态流量门禁、隔离验收身份与协议级 Gemini/Volces 模拟器，覆盖双站点 API、Worker、PostgreSQL、River、账务、回调和媒体物化链路。

新增 P24/P28/P32 容量阶梯、Worker 强杀恢复、真实小流量 canary、CAS 放量和失败保持 validation 的生产编排；Worker 执行槽、连接池、媒体并发和双站点副本数改为环境配置。

验证：Go 全量测试、真实 PostgreSQL 迁移集成测试、迁移安全检查、OpenAPI 生成、ShellCheck、Kustomize、gofmt 和 git diff --check。

apps/api/internal/httpapi/gateway_task_creation.go

@@ -44,14 +44,23 @@ func (s *Server) prepareAndCreateGatewayTask(
 	body map[string]any,
 	async bool,
 ) (store.GatewayTask, error) {
+	admission, err := s.authorizeTaskTraffic(r, user)
+	if err != nil {
+		return store.GatewayTask{}, &gatewayTaskCreationError{Stage: gatewayTaskCreationPrepare, Err: err}
+	}
 	prepared, err := s.prepareTaskRequest(ctx, r, user, body)
 	if err != nil {
 		return store.GatewayTask{}, &gatewayTaskCreationError{Stage: gatewayTaskCreationPrepare, Err: err}
 	}
+	runMode, err := s.admittedTaskRunMode(admission, prepared.Body)
+	if err != nil {
+		return store.GatewayTask{}, &gatewayTaskCreationError{Stage: gatewayTaskCreationPrepare, Err: err}
+	}
 	task, err := s.store.CreateTask(ctx, store.CreateTaskInput{
 		Kind:            kind,
 		Model:           model,
-		RunMode:         runModeFromRequest(prepared.Body),
+		RunMode:         runMode,
+		AcceptanceRunID: admission.AcceptanceRunID,
 		Async:           async,
 		Request:         prepared.Body,
 		ConversationID:  prepared.ConversationID,