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/service/setting_service.go

@@ -1272,13 +1272,10 @@ func (s *SettingService) parseSettings(settings map[string]string) *SystemSettin
 	// Account quota notification
 	result.AccountQuotaNotifyEnabled = settings[SettingKeyAccountQuotaNotifyEnabled] == "true"
 	if raw := strings.TrimSpace(settings[SettingKeyAccountQuotaNotifyEmails]); raw != "" {
-		var emails []string
-		if err := json.Unmarshal([]byte(raw), &emails); err == nil {
-			result.AccountQuotaNotifyEmails = emails
-		}
+		result.AccountQuotaNotifyEmails = ParseNotifyEmails(raw)
 	}
 	if result.AccountQuotaNotifyEmails == nil {
-		result.AccountQuotaNotifyEmails = []string{}
+		result.AccountQuotaNotifyEmails = []NotifyEmailEntry{}
 	}
 
 	return result