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_test.go

@@ -145,11 +145,13 @@ func TestProcessRolePrecedenceAndCompatibility(t *testing.T) {
 func TestValidateProcessRoleAndDatabasePool(t *testing.T) {
 	cfg := Load()
 	if cfg.DatabaseMinIdleConns != 0 ||
+		cfg.DatabaseMaxConnIdleSeconds != 0 ||
 		cfg.DatabaseIdleInTransactionTimeoutSeconds != 60 ||
 		cfg.DatabaseLockTimeoutSeconds != 30 {
 		t.Fatalf(
-			"database pool minimum/transaction timeouts = %d/%d/%d, want 0/60/30",
+			"database pool minimum/max-idle/transaction timeouts = %d/%d/%d/%d, want 0/0/60/30",
 			cfg.DatabaseMinIdleConns,
+			cfg.DatabaseMaxConnIdleSeconds,
 			cfg.DatabaseIdleInTransactionTimeoutSeconds,
 			cfg.DatabaseLockTimeoutSeconds,
 		)
@@ -174,6 +176,11 @@ func TestValidateProcessRoleAndDatabasePool(t *testing.T) {
 		t.Fatalf("Validate() error = %v, want invalid non-integer database min idle conns", err)
 	}
 	t.Setenv("AI_GATEWAY_DATABASE_MIN_IDLE_CONNS", "4")
+	cfg.DatabaseMaxConnIdleSeconds = 3601
+	if err := cfg.Validate(); err == nil || !strings.Contains(err.Error(), "DATABASE_MAX_CONN_IDLE_SECONDS") {
+		t.Fatalf("Validate() error = %v, want invalid database max connection idle seconds", err)
+	}
+	cfg.DatabaseMaxConnIdleSeconds = 30
 	cfg.DatabaseIdleInTransactionTimeoutSeconds = 3601
 	if err := cfg.Validate(); err == nil || !strings.Contains(err.Error(), "IDLE_IN_TRANSACTION") {
 		t.Fatalf("Validate() error = %v, want invalid idle transaction timeout", err)