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.

backend/internal/service/setting_service.go

@@ -1172,6 +1172,57 @@ func (s *SettingService) GetLinuxDoConnectOAuthConfig(ctx context.Context) (conf
 	return effective, nil
 }
 
+// GetOverloadCooldownSettings 获取529过载冷却配置
+func (s *SettingService) GetOverloadCooldownSettings(ctx context.Context) (*OverloadCooldownSettings, error) {
+	value, err := s.settingRepo.GetValue(ctx, SettingKeyOverloadCooldownSettings)
+	if err != nil {
+		if errors.Is(err, ErrSettingNotFound) {
+			return DefaultOverloadCooldownSettings(), nil
+		}
+		return nil, fmt.Errorf("get overload cooldown settings: %w", err)
+	}
+	if value == "" {
+		return DefaultOverloadCooldownSettings(), nil
+	}
+
+	var settings OverloadCooldownSettings
+	if err := json.Unmarshal([]byte(value), &settings); err != nil {
+		return DefaultOverloadCooldownSettings(), nil
+	}
+
+	// 修正配置值范围
+	if settings.CooldownMinutes < 1 {
+		settings.CooldownMinutes = 1
+	}
+	if settings.CooldownMinutes > 120 {
+		settings.CooldownMinutes = 120
+	}
+
+	return &settings, nil
+}
+
+// SetOverloadCooldownSettings 设置529过载冷却配置
+func (s *SettingService) SetOverloadCooldownSettings(ctx context.Context, settings *OverloadCooldownSettings) error {
+	if settings == nil {
+		return fmt.Errorf("settings cannot be nil")
+	}
+
+	// 禁用时修正为合法值即可，不拒绝请求
+	if settings.CooldownMinutes < 1 || settings.CooldownMinutes > 120 {
+		if settings.Enabled {
+			return fmt.Errorf("cooldown_minutes must be between 1-120")
+		}
+		settings.CooldownMinutes = 10 // 禁用状态下归一化为默认值
+	}
+
+	data, err := json.Marshal(settings)
+	if err != nil {
+		return fmt.Errorf("marshal overload cooldown settings: %w", err)
+	}
+
+	return s.settingRepo.Set(ctx, SettingKeyOverloadCooldownSettings, string(data))
+}
+
 // GetStreamTimeoutSettings 获取流超时处理配置
 func (s *SettingService) GetStreamTimeoutSettings(ctx context.Context) (*StreamTimeoutSettings, error) {
 	value, err := s.settingRepo.GetValue(ctx, SettingKeyStreamTimeoutSettings)