chore(PROJ-60): Xapian-Legacy-Backend vollständig aus dem Code entfernt

Xapian war seit der Manticore-Migration (PROJ-30) nur noch ein
build-tag-gatetes, in Produktion nie genutztes Legacy-Backend.
internal/index/xapian.go, xapian_stub.go, xapian_wrapper.cpp/.h entfernt;
index.New() unterstützt jetzt nur noch "manticore". Default-Fallbacks in
Nebenwerkzeugen (archivmail-import/-export, cmd_export/cmd_reindex/...)
von "xapian" auf "manticore" korrigiert, Xapian-spezifische Tests entfernt,
Kommentare bereinigt.

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

internal/index/index.go

@@ -68,7 +68,7 @@ type Indexer interface {
 // updates of the OCR-extracted attachment text. Optional add-on to Indexer:
 // callers should type-assert and degrade gracefully if not supported.
 //
-// PROJ-35: Manticore implements this; legacy Xapian does not.
+// PROJ-35: implemented by the Manticore backend.
 type AttachmentTextUpdater interface {
 	UpdateAttachmentText(mailID, text string) error
 }
@@ -82,20 +82,29 @@ type AttachmentTextReader interface {
 }
 
 // TenantIndexer manages per-tenant Indexer instances.
-// Implemented by ManticoreTenantManager (primary) and TenantIndexManager (legacy Xapian).
+// Implemented by ManticoreTenantManager.
 type TenantIndexer interface {
 	ForTenant(tenantID *int64) Indexer
 	Global() Indexer
 	Close() error
 }
 
-// New creates an Indexer for the specified backend.
-// Deprecated: use NewManticoreTenantManager instead.
+// DefaultManticoreDSN is the default Manticore connection string used when no
+// explicit DSN is configured.
+const DefaultManticoreDSN = "manticore@tcp(127.0.0.1:9306)/?charset=utf8mb4"
+
+// New creates a single (global) Indexer for the specified backend.
+// Only "manticore" is supported. For per-tenant indexing prefer
+// NewManticoreTenantManager directly.
 func New(dir string, batchSize int, backend string) (Indexer, error) {
 	switch backend {
-	case "xapian":
-		return newXapian(dir)
+	case "manticore":
+		m, err := NewManticoreTenantManager(DefaultManticoreDSN)
+		if err != nil {
+			return nil, err
+		}
+		return m.Global(), nil
 	default:
-		return nil, fmt.Errorf("unknown index backend: %q (use manticore via NewManticoreTenantManager)", backend)
+		return nil, fmt.Errorf("unknown index backend: %q (only \"manticore\" is supported)", backend)
 	}
 }