fix: preserve snapshot symlinks and guard incomplete upgrade copies

internal/api/restore_snapshot_test.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -30,7 +31,7 @@ func TestRestoreSnapshotCopyFailureDoesNotConsumeRetention(t *testing.T) {
 		}
 		configWrite(t, filepath.Join(base, name, "keep"), name)
 	}
-	if err := os.Symlink("missing-target", filepath.Join(home, "dangling")); err != nil {
+	if err := syscall.Mkfifo(filepath.Join(home, "dangling"), 0600); err != nil {
 		t.Fatal(err)
 	}
 	body := fmt.Sprintf(`{"backup_id":%q,"password":"original-password"}`, result.ID)