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/common/stubs.rs

@@ -173,6 +173,7 @@ impl FileWritePort for StubFileWritePort {
         &self,
         _file_id: &str,
         _target_folder_id: Option<String>,
+        _new_name: Option<&str>,
     ) -> Result<File, DomainError> {
         Ok(File::default())
     }
@@ -635,6 +636,7 @@ impl FileManagementUseCase for StubFileManagementUseCase {
         _file_id: &str,
         _caller_id: Uuid,
         _folder_id: Option<String>,
+        _new_name: Option<String>,
     ) -> Result<FileDto, DomainError> {
         Ok(FileDto::default())
     }