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/file_management_service.rs

@@ -127,15 +127,16 @@ impl FileManagementService {
         &self,
         file_id: &str,
         target_folder_id: Option<String>,
+        new_name: Option<&str>,
     ) -> Result<FileDto, DomainError> {
         info!(
-            "Copying file with ID: {} to folder: {:?}",
-            file_id, target_folder_id
+            "Copying file with ID: {} to folder: {:?} as {:?}",
+            file_id, target_folder_id, new_name
         );
 
         let copied_file = self
             .file_repository
-            .copy_file(file_id, target_folder_id)
+            .copy_file(file_id, target_folder_id, new_name)
             .await
             .map_err(|e| {
                 error!("Error copying file (ID: {}): {}", file_id, e);
@@ -260,13 +261,15 @@ impl FileManagementUseCase for FileManagementService {
         file_id: &str,
         caller_id: Uuid,
         target_folder_id: Option<String>,
+        new_name: Option<String>,
     ) -> Result<FileDto, DomainError> {
         // Copy = Read on the source file + Create on the target folder.
         self.require_file_perm(file_id, Permission::Read, caller_id)
             .await?;
         self.require_target_folder_perm(target_folder_id.as_deref(), Permission::Create, caller_id)
             .await?;
-        self.copy_file(file_id, target_folder_id).await
+        self.copy_file(file_id, target_folder_id, new_name.as_deref())
+            .await
     }
 
     async fn rename_file_with_perms(