perf(postgres): 回收突发连接并等待验收副作用

失败 Run 的未提交任务已取消，但退款 outbox 尚未完成时下一轮会继承 API 连接高水位和 canceled acquire 增量，污染容量验收。

新增 AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS，生产配置 30 秒回收突发空闲连接；验收中止后等待旧任务、退款、回调和数据库连接全部收敛后才允许新负载启动。

验证：Go 全量测试、gofmt、bash -n、ShellCheck、kubectl kustomize 和 git diff --check 通过。

apps/api/internal/store/postgres.go

@@ -32,6 +32,7 @@ const (
 type PostgresPoolOptions struct {
 	MaxConns                 int
 	MinIdleConns             int
+	MaxConnIdleTime          time.Duration
 	IdleInTransactionTimeout time.Duration
 	LockTimeout              time.Duration
 }
@@ -132,6 +133,9 @@ func postgresPoolConfigWithOptions(databaseURL string, options PostgresPoolOptio
 		config.MaxConns = int32(options.MaxConns)
 	}
 	config.MinIdleConns = int32(min(max(options.MinIdleConns, 0), int(config.MaxConns)))
+	if options.MaxConnIdleTime > 0 {
+		config.MaxConnIdleTime = options.MaxConnIdleTime
+	}
 	return config, nil
 }
 

apps/api/internal/store/postgres_config_test.go

@@ -61,6 +61,7 @@ func TestPostgresPoolConfigSetsBoundedTransactionTimeouts(t *testing.T) {
 		PostgresPoolOptions{
 			MaxConns:                 32,
 			MinIdleConns:             4,
+			MaxConnIdleTime:          30 * time.Second,
 			IdleInTransactionTimeout: 60 * time.Second,
 			LockTimeout:              30 * time.Second,
 		},
@@ -77,6 +78,9 @@ func TestPostgresPoolConfigSetsBoundedTransactionTimeouts(t *testing.T) {
 	if config.MaxConns != 32 || config.MinIdleConns != 4 {
 		t.Fatalf("pool bounds max=%d minIdle=%d, want 32/4", config.MaxConns, config.MinIdleConns)
 	}
+	if config.MaxConnIdleTime != 30*time.Second {
+		t.Fatalf("pool max idle time=%s, want 30s", config.MaxConnIdleTime)
+	}
 }
 
 func TestIsPostgresUnavailableClassifiesConnectivityFailures(t *testing.T) {