feat: use six-digit numeric member invite codes

internal/store/accounts.go

@@ -67,6 +67,9 @@ func (s *Store) RegisterAccount(ctx context.Context, input AccountRegistration)
 		levelID, parentID, teamLevel, tree, input.InviteCode)
 	if err != nil {
 		if isDuplicate(err) {
+			if strings.Contains(err.Error(), "uk_app_member_invite_code") {
+				return nil, ErrInviteCodeExists
+			}
 			return nil, ErrAccountExists
 		}
 		return nil, err