fix: Manticore als Standard-Backend, Xapian-Cleanup in update.sh

- main.go: Default-Backend von "xapian" auf "manticore" geändert
- index.go: Kommentar und Fehlermeldung aktualisiert
- update.sh: Xapian-Verzeichnis wird nach erfolgreichem Manticore-Reindex
  automatisch entfernt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/index/index.go

@@ -55,7 +55,7 @@ type Indexer interface {
 }
 
 // TenantIndexer manages per-tenant Indexer instances.
-// Implemented by TenantIndexManager (Xapian) and ManticoreTenantManager.
+// Implemented by ManticoreTenantManager (primary) and TenantIndexManager (legacy Xapian).
 type TenantIndexer interface {
 	ForTenant(tenantID *int64) Indexer
 	Global() Indexer
@@ -63,11 +63,12 @@ type TenantIndexer interface {
 }
 
 // New creates an Indexer for the specified backend.
+// Deprecated: use NewManticoreTenantManager instead.
 func New(dir string, batchSize int, backend string) (Indexer, error) {
 	switch backend {
 	case "xapian":
 		return newXapian(dir)
 	default:
-		return nil, fmt.Errorf("unknown index backend: %q (supported: xapian)", backend)
+		return nil, fmt.Errorf("unknown index backend: %q (use manticore via NewManticoreTenantManager)", backend)
 	}
 }