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

internal/store/errors.go

@@ -4,6 +4,7 @@ import "errors"
 
 var (
 	ErrAccountExists         = errors.New("account already exists")
+	ErrInviteCodeExists      = errors.New("invite code already exists")
 	ErrAccountNotFound       = errors.New("account not found")
 	ErrAccountRestricted     = errors.New("account restricted")
 	ErrAttachmentInvalid     = errors.New("attachment invalid")