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',