refactor: replace KeyMode with ShouldPersistKey

apps/backend/internal/virtualfs/key_resolver.go

@@ -7,7 +7,10 @@ import (
 )
 
 type BlobKeyResolver interface {
-	KeyMode() blob.KeyMode
+	// ShouldPersistKey returns true if the resolved key should be stored in node.BlobKey.
+	// Flat keys (e.g. UUIDs) return true - key is generated once and stored.
+	// Hierarchical keys return false - key is derived from path each time.
+	ShouldPersistKey() bool
 	Resolve(ctx context.Context, node *Node) (blob.Key, error)
 	ResolveDeletionKeys(ctx context.Context, node *Node, allKeys []blob.Key) (*DeletionPlan, error)
 }