fix: add missing AccountQuotaNotifyEnabled to admin settings API

The field was present in SystemSettings response DTO and service layer
but missing from:
- UpdateSettingsRequest (admin handler) - saves were silently ignored
- GET/PUT response mapping in admin handler
- UpdateSettingsRequest (non-admin dto)

This caused the toggle to always revert to off after saving.

backend/internal/handler/dto/settings.go

@@ -152,6 +152,7 @@ type SystemSettings struct {
 	// Balance low notification
 	BalanceLowNotifyEnabled   bool     `json:"balance_low_notify_enabled"`
 	BalanceLowNotifyThreshold float64  `json:"balance_low_notify_threshold"`
+	AccountQuotaNotifyEnabled bool     `json:"account_quota_notify_enabled"`
 	AccountQuotaNotifyEmails  []NotifyEmailEntry `json:"account_quota_notify_emails"`
 }