fix(acceptance): 隔离容量压测并稳定数据库连接

将协议模拟验收的供应商并发限额与 Worker 容量门禁分离，真实金丝雀继续保留生产限流。readyz 改用关键连接池并预热关键及 River 池，延长生产连接空闲周期，降低跨地域连接抖动。失败 Run 现在可以原子替换且失败报告记录任务数量，避免 validation 之间短暂放开正式流量。\n\n验证：Go 全量测试、go vet、PostgreSQL 集成测试、ShellCheck、迁移安全、发布脚本、pnpm lint/test/build、OpenAPI 无漂移均通过。

apps/api/internal/store/acceptance.go

@@ -209,9 +209,6 @@ FOR UPDATE`, SystemSettingGatewayTrafficMode).Scan(&currentValue); err != nil {
 	if err := json.Unmarshal(currentValue, &current); err != nil {
 		return GatewayTrafficMode{}, err
 	}
-	if normalizeTrafficMode(current.Mode) != "live" {
-		return GatewayTrafficMode{}, ErrAcceptanceStateConflict
-	}
 	run, err := scanAcceptanceRun(tx.QueryRow(ctx, `
 SELECT `+acceptanceRunColumns+`
 FROM gateway_acceptance_runs
@@ -223,6 +220,40 @@ FOR UPDATE`, strings.TrimSpace(runID)))
 	if run.Status != "pending" && run.Status != "failed" {
 		return GatewayTrafficMode{}, ErrAcceptanceStateConflict
 	}
+	switch normalizeTrafficMode(current.Mode) {
+	case "live":
+	case "validation":
+		if current.RunID == "" || current.RunID == run.ID {
+			return GatewayTrafficMode{}, ErrAcceptanceStateConflict
+		}
+		previous, previousErr := scanAcceptanceRun(tx.QueryRow(ctx, `
+SELECT `+acceptanceRunColumns+`
+FROM gateway_acceptance_runs
+WHERE id = $1::uuid
+FOR UPDATE`, current.RunID))
+		if previousErr != nil {
+			return GatewayTrafficMode{}, previousErr
+		}
+		if previous.Status != "failed" ||
+			previous.ReleaseSHA != current.ReleaseSHA ||
+			previous.APIImageDigest != current.APIImageDigest ||
+			previous.WorkerImageDigest != current.WorkerImageDigest {
+			return GatewayTrafficMode{}, ErrAcceptanceStateConflict
+		}
+		var activeTasks int
+		if err := tx.QueryRow(ctx, `
+SELECT count(*)
+FROM gateway_tasks
+WHERE acceptance_run_id = $1::uuid
+  AND status IN ('queued', 'running')`, previous.ID).Scan(&activeTasks); err != nil {
+			return GatewayTrafficMode{}, err
+		}
+		if activeTasks != 0 {
+			return GatewayTrafficMode{}, ErrAcceptanceStateConflict
+		}
+	default:
+		return GatewayTrafficMode{}, ErrAcceptanceStateConflict
+	}
 	next := GatewayTrafficMode{
 		Mode:              "validation",
 		RunID:             run.ID,