fix(downloads): accept optional completed_at in complete_download to preserve original timestamps

py/services/download_queue_service.py

@@ -349,6 +349,7 @@ class DownloadQueueService:
         file_path: Optional[str] = None,
         bytes_downloaded: int = 0,
         total_bytes: Optional[int] = None,
+        completed_at: Optional[float] = None,
     ) -> Optional[dict[str, Any]]:
         """Atomically move a download from the queue into the history table.
 
@@ -356,6 +357,9 @@ class DownloadQueueService:
         queue, and inserts a corresponding history entry with the given
         terminal status (``completed``, ``failed``, or ``canceled``).
 
+        When *completed_at* is provided it is used as the completion
+        timestamp; otherwise ``time.time()`` is used.
+
         Returns the original queue record (before deletion) on success,
         or ``None`` if the download was not found in the queue.
         """
@@ -368,7 +372,7 @@ class DownloadQueueService:
             if row is None:
                 return None
 
-            now = time.time()
+            now = completed_at if completed_at is not None else time.time()
             conn.execute(
                 "DELETE FROM download_queue WHERE download_id = ?",
                 (download_id,),