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/settings_view.go

@@ -222,6 +222,22 @@ type BetaPolicySettings struct {
 	Rules []BetaPolicyRule `json:"rules"`
 }
 
+// OverloadCooldownSettings 529过载冷却配置
+type OverloadCooldownSettings struct {
+	// Enabled 是否在收到529时暂停账号调度
+	Enabled bool `json:"enabled"`
+	// CooldownMinutes 冷却时长（分钟）
+	CooldownMinutes int `json:"cooldown_minutes"`
+}
+
+// DefaultOverloadCooldownSettings 返回默认的过载冷却配置（启用，10分钟）
+func DefaultOverloadCooldownSettings() *OverloadCooldownSettings {
+	return &OverloadCooldownSettings{
+		Enabled:         true,
+		CooldownMinutes: 10,
+	}
+}
+
 // DefaultBetaPolicySettings 返回默认的 Beta 策略配置
 func DefaultBetaPolicySettings() *BetaPolicySettings {
 	return &BetaPolicySettings{