fix(app-assets): restore atomic batch upload for nested folder targets

The previous nested folder upload flow bypassed the backend batch-upload
contract when parentId was set. Instead of creating the whole metadata
tree in one backend operation, the frontend recursively called
createFolder/getFileUploadUrl for each node.

That introduced two regressions for uploads into subfolders:

- consistency regression: mid-sequence failures could leave partially
  created folder trees under the destination folder
- performance regression: metadata creation degraded from a single
  batch request to O(files + folders) round-trips before file bytes
  were uploaded

This change moves nested uploads back to the original batch semantics:

- add optional parent_id support to app asset batch-upload payload
- create the whole nested tree under the target parent in
  AppAssetService.batch_create_from_tree
- pass parentId through useBatchUpload instead of using per-node
  createFolder/getFileUploadUrl calls
- remove the now-unnecessary useBatchUploadOperation wrapper
- add a backend unit test covering batch tree creation under an
  existing parent folder

After this change, both root uploads and subfolder uploads use the same
single-request metadata creation path, preserving atomic tree creation
semantics and avoiding avoidable metadata round-trips.

web/types/app-asset.ts

@@ -179,6 +179,7 @@ export type BatchUploadNodeOutput = {
  */
 export type BatchUploadPayload = {
   children: BatchUploadNodeInput[]
+  parent_id?: string | null
 }
 
 /**