[management] resolve private services on custom domains in synthesized DNS zones (#6348)

private services on a custom domain didn't resolve on clients — the synthesized DNS zone was anchored to the cluster, and the account's custom domains weren't even
  loaded.

- account.go — SynthesizePrivateServiceZones now keys zones by a resolved apex (privateServiceDomainZone): cluster suffix → registered account.Domains (filtered by matching
  TargetCluster, longest wins) → skip if none. One zone per apex; custom-domain services group under their registered domain.
- sql_store.go — GetAccount now loads account.Domains on both loaders (gorm Preload("Domains") + pgx goroutine via ListCustomDomains; errChan buffer bumped 12→16). This was
  the reason the deploy didn't work — the relation was empty in prod.
- Tests — custom-domain zone synthesis cases (apex resolution, free+custom separation, sibling collapse, cluster mismatch, mixed cluster/custom/public) + GetAccount
  domain-preload tests on sqlite and Postgres.

management/server/store/sql_store.go

@@ -1216,6 +1216,7 @@ func (s *SqlStore) getAccountGorm(ctx context.Context, accountID string) (*types
 		Preload("NetworkResources").
 		Preload("Onboarding").
 		Preload("Services.Targets").
+		Preload("Domains").
 		Take(&account, idQueryCondition, accountID)
 	if result.Error != nil {
 		log.WithContext(ctx).Errorf("error when getting account %s from the store: %s", accountID, result.Error)
@@ -1302,7 +1303,7 @@ func (s *SqlStore) getAccountPgx(ctx context.Context, accountID string) (*types.
 	}
 
 	var wg sync.WaitGroup
-	errChan := make(chan error, 12)
+	errChan := make(chan error, 16)
 
 	wg.Add(1)
 	go func() {
@@ -1403,6 +1404,17 @@ func (s *SqlStore) getAccountPgx(ctx context.Context, accountID string) (*types.
 		account.Services = services
 	}()
 
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		domains, err := s.ListCustomDomains(ctx, accountID)
+		if err != nil {
+			errChan <- err
+			return
+		}
+		account.Domains = domains
+	}()
+
 	wg.Add(1)
 	go func() {
 		defer wg.Done()