Back up symlinks, empty directories, and file permissions

Scanner now records symlinks (with their target) and directories
during the walk phase instead of skipping them. processFileStreaming
detects non-regular entries and writes the DB record without chunking.

The e2e test (TestEndToEndFileStorage) now verifies:
- Symlink target preserved through backup→restore
- Empty directory survives round-trip
- File permissions (0600) restored correctly

internal/snapshot/scanner_test.go

@@ -110,15 +110,15 @@ func TestScannerSimpleDirectory(t *testing.T) {
 		t.Errorf("expected at least 97 bytes scanned, got %d", result.BytesScanned)
 	}
 
-	// Verify files in database - only regular files are stored
+	// Verify files in database - includes regular files and directories
 	files, err := repos.Files.ListByPrefix(ctx, "/source")
 	if err != nil {
 		t.Fatalf("failed to list files: %v", err)
 	}
 
-	// We should have 6 files (directories are not stored)
-	if len(files) != 6 {
-		t.Errorf("expected 6 files in database, got %d", len(files))
+	// 6 regular files + 3 directories (/source, /source/subdir, /source/subdir2)
+	if len(files) != 9 {
+		t.Errorf("expected 9 entries in database (6 files + 3 dirs), got %d", len(files))
 	}
 
 	// Verify specific file