feat(tmpl): {{domain_name}} placeholder — materialize on diff/apply, parameterize on snapshot

Adds internal/tmpl with Materialize (template placeholder -> zone name) and
Parameterize (zone name -> placeholder, the inverse used by the
template-from-zone snapshot). service.resolve now materializes the template
against DomainRef.ZoneName before diffing, so one template can be reused
across domains. LoadDomainFull (source query + hand-edited sqlc output, since
sqlc is not installed) now also selects zone_name to populate it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

internal/store/loader.go

@@ -27,6 +27,7 @@ func (s *Store) LoadDomain(ctx context.Context, projectID, domainID uuid.UUID) (
 	}
 	return service.DomainRef{
 		ZoneID:    row.ZoneID,
+		ZoneName:  row.ZoneName,
 		Provider:  row.Provider,
 		SecretEnc: row.SecretEnc,
 		Template:  *row.Doc,