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/file_ports.rs

@@ -255,11 +255,17 @@ pub trait FileManagementUseCase: Send + Sync + 'static {
     ) -> Result<FileDto, DomainError>;
 
     /// Copies a file, enforcing that `caller_id` is the owner.
+    ///
+    /// `new_name`, when `Some(_)`, becomes the copy's filename — without it
+    /// the copy keeps the source's name, which makes "same folder, different
+    /// name" copies (classic WebDAV `COPY /a.txt → /b.txt`) collide on the
+    /// `(folder, name, user)` unique index.
     async fn copy_file_with_perms(
         &self,
         file_id: &str,
         caller_id: Uuid,
         target_folder_id: Option<String>,
+        new_name: Option<String>,
     ) -> Result<FileDto, DomainError>;
 
     /// Renames a file, enforcing that `caller_id` is the owner.

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.

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)
             }

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(

src/application/services/idor_protection_test.rs

@@ -228,6 +228,7 @@ impl FileWritePort for MockFileWritePort {
         &self,
         _file_id: &str,
         _target_folder_id: Option<String>,
+        _new_name: Option<&str>,
     ) -> Result<File, DomainError> {
         unimplemented!()
     }

src/application/services/trash_service_test.rs

@@ -609,6 +609,7 @@ impl FileWritePort for MockFileRepository {
         &self,
         _file_id: &str,
         _target_folder_id: Option<String>,
+        _new_name: Option<&str>,
     ) -> std::result::Result<File, DomainError> {
         unimplemented!()
     }