feat: integrate native queue backend into worker and API

- Add QueueBackendNative constant to backend.go
- Add case for native queue in NewBackend() switch
- Native queue uses same FilesystemPath config
- Build tag -tags native_libs enables native implementation

Native library integration now complete:
- dataset_hash: Worker (hash_selector), CLI (verify auto-hash)
- queue_index: Worker/API (backend selection with 'native' type)

internal/queue/backend.go

@@ -51,6 +51,7 @@ const (
 	QueueBackendRedis  QueueBackend = "redis"
 	QueueBackendSQLite QueueBackend = "sqlite"
 	QueueBackendFS     QueueBackend = "filesystem"
+	QueueBackendNative QueueBackend = "native" // Native C++ queue_index (requires -tags native_libs)
 )
 
 type BackendConfig struct {
@@ -85,6 +86,11 @@ func NewBackend(cfg BackendConfig) (Backend, error) {
 			return nil, fmt.Errorf("filesystem queue path is required")
 		}
 		return NewFilesystemQueue(cfg.FilesystemPath)
+	case QueueBackendNative:
+		if strings.TrimSpace(cfg.FilesystemPath) == "" {
+			return nil, fmt.Errorf("native queue path is required")
+		}
+		return NewNativeQueue(cfg.FilesystemPath)
 	case "", QueueBackendRedis:
 		b, err := NewTaskQueue(Config{
 			RedisAddr:            cfg.RedisAddr,