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/config/config.go

@@ -60,6 +60,7 @@ type Config struct {
 	ProcessRole                                   string
 	DatabaseMaxConns                              int
 	DatabaseMinIdleConns                          int
+	DatabaseMaxConnIdleSeconds                    int
 	DatabaseIdleInTransactionTimeoutSeconds       int
 	DatabaseLockTimeoutSeconds                    int
 	MediaRequestConcurrency                       int
@@ -124,6 +125,7 @@ func Load() Config {
 		ProcessRole:                     strings.ToLower(strings.TrimSpace(env("AI_GATEWAY_PROCESS_ROLE", "all"))),
 		DatabaseMaxConns:                envInt("AI_GATEWAY_DATABASE_MAX_CONNS", 0),
 		DatabaseMinIdleConns:            envOptionalIntValidated("AI_GATEWAY_DATABASE_MIN_IDLE_CONNS", 0),
+		DatabaseMaxConnIdleSeconds:      envOptionalIntValidated("AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS", 0),
 
 		DatabaseIdleInTransactionTimeoutSeconds: envOptionalIntValidated(
 			"AI_GATEWAY_DATABASE_IDLE_IN_TRANSACTION_TIMEOUT_SECONDS",
@@ -153,6 +155,9 @@ func (c Config) Validate() error {
 		(c.DatabaseMaxConns > 0 && c.DatabaseMinIdleConns > c.DatabaseMaxConns) {
 		return errors.New("AI_GATEWAY_DATABASE_MIN_IDLE_CONNS must be between 0 and AI_GATEWAY_DATABASE_MAX_CONNS")
 	}
+	if c.DatabaseMaxConnIdleSeconds < 0 || c.DatabaseMaxConnIdleSeconds > 3600 {
+		return errors.New("AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS must be between 0 and 3600")
+	}
 	if c.DatabaseIdleInTransactionTimeoutSeconds < 0 || c.DatabaseIdleInTransactionTimeoutSeconds > 3600 {
 		return errors.New("AI_GATEWAY_DATABASE_IDLE_IN_TRANSACTION_TIMEOUT_SECONDS must be between 0 and 3600")
 	}