feat(notify): convert email lists to NotifyEmailEntry struct with toggle support

- Change balance_notify_extra_emails and account_quota_notify_emails
  from []string to []NotifyEmailEntry{email, disabled, verified}
- Add per-email enable/disable toggle for both user and admin notifications
- Add PUT /user/notify-email/toggle API endpoint
- Fix critical bug: API key auth cache snapshot missing balance notify
  fields (Email, Username, BalanceNotifyEnabled, etc.), causing
  notifications to never fire on cached request paths
- Bump cache snapshot version 3→4 to invalidate stale entries
- Add SQL migration 104 to convert old format data
- Backward compatible: parseNotifyEmails auto-detects old/new format
- User balance notify: max 3 emails (primary + 2 extra)
- Admin quota notify: unlimited emails, each with toggle

backend/internal/handler/dto/mappers.go

@@ -26,7 +26,7 @@ func UserFromServiceShallow(u *service.User) *User {
 		BalanceNotifyEnabled:       u.BalanceNotifyEnabled,
 		BalanceNotifyThresholdType: u.BalanceNotifyThresholdType,
 		BalanceNotifyThreshold:     u.BalanceNotifyThreshold,
-		BalanceNotifyExtraEmails:   u.BalanceNotifyExtraEmails,
+		BalanceNotifyExtraEmails:   NotifyEmailEntriesFromService(u.BalanceNotifyExtraEmails),
 		TotalRecharged:             u.TotalRecharged,
 	}
 }