fix: add ResolveRenameOp to handle directory renames

RenameNode was calling Resolve() which generated blob keys for
directories that don't have blobs, causing 'key not found' errors.

Added ResolveRenameOp to BlobKeyResolver interface:
- FlatKeyResolver returns nil (UUIDs don't change on rename)
- HierarchicalKeyResolver returns move op for files and directories

This allows directory renames to work correctly with flat storage,
and leverages os.Rename for atomic directory moves with hierarchical.

Co-authored-by: Ona <no-reply@ona.com>

apps/backend/internal/virtualfs/flat_key_resolver.go

@@ -45,3 +45,9 @@ func (r *FlatKeyResolver) ResolveBulkMoveOps(ctx context.Context, db bun.IDB, no
 	}
 	return nil, nil
 }
+
+// ResolveRenameOp returns nil for flat key storage since blob keys are UUIDs
+// and don't change when nodes are renamed.
+func (r *FlatKeyResolver) ResolveRenameOp(ctx context.Context, db bun.IDB, node *Node, newName string) (*BlobMoveOp, error) {
+	return nil, nil
+}