fix(files): a folder's kind is answered before its new name, as before

A blank name on a source folder answered 400 name_required; it answers 409
not_own again.

backend/app/downloads/folders.py

@@ -328,9 +328,14 @@ def create_own(db: Session, user_id: int, name: str) -> FileFolder:
     return folder
 
 
+def renamable(folder: FileFolder) -> bool:
+    """Only a folder of the user's own making takes a name: a source folder's is its origin's."""
+    return folder.kind == FolderKind.OWN
+
+
 def rename(db: Session, folder: FileFolder, name: str) -> None:
     """Rename one of the user's own folders (a source folder's name is its origin's)."""
-    if folder.kind != FolderKind.OWN:
+    if not renamable(folder):
         raise FolderError("file_folder_not_own")
     folder.name = name
     try:

backend/app/routes/files.py

@@ -375,6 +375,9 @@ def update_folder(
     folder = own_folder(db, user, folder_id)
     sent = body.model_fields_set
     if "name" in sent:
+        # Which folder may be renamed is answered before what the name is, as it always was.
+        if not folders.renamable(folder):
+            raise refused(409, "file_folder_not_own")
         try:
             folders.rename(db, folder, _folder_name(body.name or ""))
         except folders.FolderError as e: