fix(webdav): native COPY honours destination filename (M8)

Threads `new_name: Option<&str>` through FileWritePort::copy_file and
FileManagementUseCase::copy_file_with_perms so a same-folder
COPY /a.txt → /b.txt picks up the destination name via a single
COALESCE($3::text, name) in the CTE. Without it the new row inherits
the source's filename and collides on the (folder_id, name, user_id)
unique index — the "Already Exists" 500 M8 was hitting.

handle_copy in the native WebDAV surface now passes
`(file.name != dest_name).then(|| dest_name.into())`, keeping the
"same name in a different folder" case at None so existing semantics
are preserved.

src/application/services/batch_operations.rs

@@ -145,7 +145,12 @@ impl BatchOperationService {
 
             async move {
                 let copy_result = mgmt
-                    .copy_file_with_perms(&file_id, user_id, target_folder.map(|s| s.to_string()))
+                    .copy_file_with_perms(
+                        &file_id,
+                        user_id,
+                        target_folder.map(|s| s.to_string()),
+                        None,
+                    )
                     .await;
                 (file_id, copy_result)
             }