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。

deploy/kubernetes/production/application.yaml

@@ -56,6 +56,8 @@ spec:
               value: "64"
             - name: AI_GATEWAY_DATABASE_MIN_IDLE_CONNS
               value: "4"
+            - name: AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS
+              value: "30"
             - name: AI_GATEWAY_MEDIA_REQUEST_CONCURRENCY
               value: "16"
             - name: AI_GATEWAY_MEDIA_MATERIALIZATION_CONCURRENCY
@@ -187,6 +189,8 @@ spec:
               value: "64"
             - name: AI_GATEWAY_DATABASE_MIN_IDLE_CONNS
               value: "4"
+            - name: AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS
+              value: "30"
             - name: AI_GATEWAY_MEDIA_REQUEST_CONCURRENCY
               value: "16"
             - name: AI_GATEWAY_MEDIA_MATERIALIZATION_CONCURRENCY
@@ -318,6 +322,8 @@ spec:
               value: "32"
             - name: AI_GATEWAY_DATABASE_MIN_IDLE_CONNS
               value: "4"
+            - name: AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS
+              value: "30"
             - name: AI_GATEWAY_ASYNC_WORKER_INSTANCE_HARD_LIMIT
               value: "24"
             - name: AI_GATEWAY_MEDIA_MATERIALIZATION_CONCURRENCY
@@ -451,6 +457,8 @@ spec:
               value: "32"
             - name: AI_GATEWAY_DATABASE_MIN_IDLE_CONNS
               value: "4"
+            - name: AI_GATEWAY_DATABASE_MAX_CONN_IDLE_SECONDS
+              value: "30"
             - name: AI_GATEWAY_ASYNC_WORKER_INSTANCE_HARD_LIMIT
               value: "24"
             - name: AI_GATEWAY_MEDIA_MATERIALIZATION_CONCURRENCY