fix(rename): refresh Folder.name on webhook-driven directory rename

handle_directory_rename updated Folder.path but left Folder.name as
the old leaf basename. Path is load-bearing; name is purely display,
but a stale name shows wrong text in the sidebar tree until the next
manual refresh. Now sets folder.name = basename(new_prefix) on the
renamed folder itself; descendants keep their existing names because
the rename was on an ancestor (only their paths shift).

Same correctness as the existing PATCH /folders/{id} endpoint, which
also updates both name and path.

backend/app/tasks/scan.py

@@ -641,9 +641,18 @@ async def handle_directory_rename(old_dirpath: str, new_dirpath: str) -> dict:
                 )
             )
         )).scalars().all()
+        # Path is load-bearing (FKs join on it implicitly via filepath),
+        # name is purely display. The renamed folder itself gets its
+        # leaf basename refreshed too so the sidebar tree doesn't show
+        # stale text. Descendant folders keep their existing name
+        # because the rename was on the ancestor — only the path changes.
+        new_basename = os.path.basename(new_prefix)
         for f in folders:
-            f.path = new_prefix if f.path == old_prefix else \
-                new_prefix + f.path[len(old_prefix):]
+            if f.path == old_prefix:
+                f.path = new_prefix
+                f.name = new_basename
+            else:
+                f.path = new_prefix + f.path[len(old_prefix):]
 
         source_roots = (await session.execute(
             select(SourceRoot).where(SourceRoot.path == old_prefix)