[management] Refuse services on unvalidated custom domains

A custom domain row was bound to a live service whether or not its CNAME
validation ever succeeded: extractClusterFromCustomDomains matched on the name
alone, so an account that never proved DNS control still had its hostname
routed. Cluster derivation now matches only validated rows and reports "domain
is not validated" instead of the generic no-cluster message.

Service updates no longer fall back to the previously derived cluster when
derivation fails, which was a way around the same check.

The unique index on the domain column already prevents two accounts holding the
same name, but the violation surfaced as an internal error. Creation now
pre-checks and returns AlreadyExists so the caller gets a 409, and the message
does not say which account holds the domain.

management/server/store/sql_store.go

@@ -5658,6 +5658,23 @@ func (s *SqlStore) ListCustomDomains(ctx context.Context, accountID string) ([]*
 	return domains, nil
 }
 
+// GetCustomDomainByName returns the custom domain row holding the given name,
+// regardless of which account owns it.
+func (s *SqlStore) GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error) {
+	customDomain := &domain.Domain{}
+	result := s.db.Take(customDomain, "domain = ?", domainName)
+	if result.Error != nil {
+		if errors.Is(result.Error, gorm.ErrRecordNotFound) {
+			return nil, status.Errorf(status.NotFound, "custom domain %s not found", domainName)
+		}
+
+		log.WithContext(ctx).Errorf("failed to get custom domain by name from store: %v", result.Error)
+		return nil, status.Errorf(status.Internal, "failed to get custom domain from store")
+	}
+
+	return customDomain, nil
+}
+
 func (s *SqlStore) CreateCustomDomain(ctx context.Context, accountID string, domainName string, targetCluster string, validated bool) (*domain.Domain, error) {
 	newDomain := &domain.Domain{
 		ID:            xid.New().String(), // Generate our own ID because gorm doesn't always configure the database to handle this for us.

management/server/store/store.go

@@ -294,6 +294,7 @@ type Store interface {
 	GetCustomDomain(ctx context.Context, accountID string, domainID string) (*domain.Domain, error)
 	ListFreeDomains(ctx context.Context, accountID string) ([]string, error)
 	ListCustomDomains(ctx context.Context, accountID string) ([]*domain.Domain, error)
+	GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error)
 	CreateCustomDomain(ctx context.Context, accountID string, domainName string, targetCluster string, validated bool) (*domain.Domain, error)
 	UpdateCustomDomain(ctx context.Context, accountID string, d *domain.Domain) (*domain.Domain, error)
 	DeleteCustomDomain(ctx context.Context, accountID string, domainID string) error

management/server/store/store_mock.go

@@ -1892,6 +1892,21 @@ func (mr *MockStoreMockRecorder) GetCustomDomain(ctx, accountID, domainID interf
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetCustomDomain", reflect.TypeOf((*MockStore)(nil).GetCustomDomain), ctx, accountID, domainID)
 }
 
+// GetCustomDomainByName mocks base method.
+func (m *MockStore) GetCustomDomainByName(ctx context.Context, domainName string) (*domain.Domain, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetCustomDomainByName", ctx, domainName)
+	ret0, _ := ret[0].(*domain.Domain)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetCustomDomainByName indicates an expected call of GetCustomDomainByName.
+func (mr *MockStoreMockRecorder) GetCustomDomainByName(ctx, domainName interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetCustomDomainByName", reflect.TypeOf((*MockStore)(nil).GetCustomDomainByName), ctx, domainName)
+}
+
 // GetCustomDomainsCounts mocks base method.
 func (m *MockStore) GetCustomDomainsCounts(ctx context.Context) (int64, int64, error) {
 	m.ctrl.T.Helper()