feat: add 529 overload cooldown toggle and duration settings in admin gateway page

Move 529 overload cooldown configuration from config file to admin
settings UI. Adds an enable/disable toggle and configurable cooldown
duration (1-120 min) under /admin/settings gateway tab, stored as
JSON in the settings table.

When disabled, 529 errors are logged but accounts are no longer
paused from scheduling. Falls back to config file value when DB
is unreachable or settingService is nil.

frontend/src/i18n/locales/en.ts

@@ -4362,6 +4362,16 @@ export default {
           testFailed: 'Google Drive storage test failed'
         }
       },
+      overloadCooldown: {
+        title: '529 Overload Cooldown',
+        description: 'Configure account scheduling pause strategy when upstream returns 529 (overloaded)',
+        enabled: 'Enable Overload Cooldown',
+        enabledHint: 'Pause account scheduling on 529 errors, auto-recover after cooldown',
+        cooldownMinutes: 'Cooldown Duration (minutes)',
+        cooldownMinutesHint: 'Duration to pause account scheduling (1-120 minutes)',
+        saved: 'Overload cooldown settings saved',
+        saveFailed: 'Failed to save overload cooldown settings'
+      },
       streamTimeout: {
         title: 'Stream Timeout Handling',
         description: 'Configure account handling strategy when upstream response times out',

frontend/src/i18n/locales/zh.ts

@@ -4527,6 +4527,16 @@ export default {
           testFailed: 'Google Drive 存储测试失败'
         }
       },
+      overloadCooldown: {
+        title: '529 过载冷却',
+        description: '配置上游返回 529（过载）时的账号调度暂停策略',
+        enabled: '启用过载冷却',
+        enabledHint: '收到 529 错误时暂停该账号的调度，冷却后自动恢复',
+        cooldownMinutes: '冷却时长（分钟）',
+        cooldownMinutesHint: '账号暂停调度的持续时间（1-120 分钟）',
+        saved: '过载冷却设置保存成功',
+        saveFailed: '保存过载冷却设置失败'
+      },
       streamTimeout: {
         title: '流超时处理',
         description: '配置上游响应超时时的账户处理策略，避免问题账户持续被选中',