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/ports/storage_ports.rs

@@ -314,10 +314,16 @@ pub trait FileWritePort: Send + Sync + 'static {
     ///
     /// With blob-dedup, this only creates a new metadata row and increments
     /// the blob reference count — zero disk I/O for the content.
+    ///
+    /// `new_name` is honored when `Some(_)` — without it, copying a file to
+    /// the same folder always collides on the source's filename. WebDAV
+    /// COPY uses this for the "same folder, different name" case (the
+    /// classic `COPY /a.txt → /b.txt` pattern).
     async fn copy_file(
         &self,
         file_id: &str,
         target_folder_id: Option<String>,
+        new_name: Option<&str>,
     ) -> Result<File, DomainError>;
 
     /// Copies an entire folder subtree atomically using ltree.